Hilsea Lines


Heritage Category:
Scheduled Monument
List Entry Number:
Date first listed:
Date of most recent amendment:
Location Description:
Statutory Address:
North end of Hilsea Island, Portsmouth, PO3 5JH


© Crown Copyright and database right 2021. All rights reserved. Ordnance Survey Licence number 100024900.
© British Crown and SeaZone Solutions Limited 2021. All rights reserved. Licence number 102006.006.
Use of this data is subject to Terms and Conditions.

The above map is for quick reference purposes only and may not be to scale. For a copy of the full scale map, please see the attached PDF - 1001861.pdf

The PDF will be generated from our live systems and may take a few minutes to download depending on how busy our servers are. We apologise for this delay.

This copy shows the entry on 27-Feb-2021 at 13:12:27.


Statutory Address:
North end of Hilsea Island, Portsmouth, PO3 5JH

The building or site itself may lie within the boundary of more than one authority.

Location Description:
City of Portsmouth (Unitary Authority)
Non Civil Parish
National Grid Reference:


Linear bastioned and casemated defensive fortification, built between 1858 and 1871, replacing C18 defences protecting the northern approach to Portsmouth.

Reasons for Designation

Hilsea Lines, a bastion-trace defensive line built between 1858 and 1871 replacing earlier C18 fortifications defending the northern approaches to Portsmouth Harbour, is scheduled for the following principal reasons:

* Rarity: as the last true bastioned and casemated defensive line built in England;

* Historic interest: as an immediate predecessor to the massive programme of defensive works stemming from the 1860 Royal Commission and prompted by the same fears of a French invasion. The evidence of later rearmament in the form of Moncrieff gun pits illustrates the reaction to the rapid technological development of artillery in the late-C19. The Hilsea Lines form an important part of the defences of Portsmouth, the oldest, most extensive and best surviving group of dockyard defences in the country; * Survival: it provides a good example of the construction and form of a substantial mid-C19 ditched earthwork defence with brick casemates, magazines and sallyport which all survive well. The Lines are a major work of the prominent Victorian military engineer Sir William Jervois;

* Documentation: the site is well documented including written accounts, historic plans, maps and photographs, as well as recent condition surveys;

* Potential: for archaeological deposits associated with the construction and use of the Lines. In addition, buried remains or artefactual evidence relating to the C18 defences may also survive;

* Group value: with the other designated fortifications associated with Portsmouth Harbour.


The Hilsea Lines were constructed between 1858 and 1871 and were the last of a series of fortifications built at the northern end of Portsea Island, protecting the land approaches to the Portsmouth dockyards from the north. The defence of Portsea Island is believed to have originated with a fortification (the Portway bulwark) erected in the reign of Henry VIII on the north bank of Port (or Portsbridge) Creek to protect its bridge. A fortification on the site was defended by Royalist forces in 1642 during the Civil War. At the Restoration, under Charles II, the Dutch military engineer, Bernard de Gomme (1620-1685) began the reconstruction of Portsmouth’s fortifications and plans show the fort defending the bridge, dated 1660 and 1666. In 1688 under Martin Beckman (1634/5-1702), the fortification at the Port Creek crossing was rebuilt. In the C18, with the abandonment of de Gomme’s fortifications around Portsmouth itself in 1730 and the increased development of Portsea, there was a need for new and stronger fortifications at the north end of the island. Between 1756 and 1757 defensive lines were constructed by John Peter Desmaretz (c1686-1768), Clerk to the Fortifications for the Portsmouth Dockyard. He was also responsible for building the Gosport Lines on the other side of Portsmouth harbour around the same time. The Hilsea Lines ran along the south bank of Port Creek (with an outwork on the north bank guarding the bridge, which had been rebuilt again in 1746). The Lines consisted of an irregular zigzag trace with three principal batteries and a single magazine. It had a 4.6m to 6m wide, 2m deep, ditch and a 2m to 2.4m earth rampart. Apart from the addition of a new work known as the Hilsea Redoubt, incorporated into the Lines when they were pierced by a railway line in 1846, and the addition of four expense magazines in 1853, the lines remained largely unchanged for 100 years.

By the late-1850s a combination of endemic mistrust of France and resultant fear of a potential invasion, developments in military technology, lessons learned from the siege of Sebastopol during the Crimean War, and criticism regarding the poor state of Portsmouth’s defences, lead to calls for a review of the dockyard’s fortifications. Consequently in 1857 Colonel WFD Jervois (1821-1897) of the Royal Engineers submitted plans for ’completing the defence works of Portsmouth’. This concluded that existing defences were too close to the dockyard, were too small and had fields of fire masked by buildings erected outside the lines. The Hilsea Lines themselves were considered to be ‘of weak trace and low profile’ and it was proposed, along with recommendations for other improvements for the overall defence of Portsmouth and Gosport, that they be reconstructed. Work on the Hilsea Lines and Gosport forts began in 1858. In addition to the rebuilding, batteries on Horsea Island and Shut Point to protect the flanks of the Lines and a further work on Pewitt Island to link the Portchester and Gosport lines were proposed. The design for the new Lines consisted of a bastioned trace on the south bank of Port Creek consisting of earth ramparts fronted by a water-filled ditch with three full bastions and two demi-bastions equipped with brick Haxo casemates, housing ten and five guns respectively (apart from the eastern flank of the East Bastion and the East Demi-Bastion which, considered less liable to attack, were not to have casemates). On the north bank were three outworks, the outer two of which protected the road (which was repositioned) and railway bridges. These were later linked in revised plans. Two small forts to the rear of the Lines were included in the plans but never built. Port Creek was straightened, widened and deepened, with dams added at either end to make it non-tidal.

Approval for the construction of the new Lines was obtained in January 1858 and William Piper of London, was awarded the first contract for the demolition of the old lines and the building of the casemates and the entrance gateways. By 1859, however, the development of the Armstrong rifled breech-loading gun with increased accuracy and range led Jervois to rethink his proposals fearing that a force which occupied Portsdown Hill could now bombard the dockyard and Lines from long range. He proposed that the Hilea Lines be abandoned and that a new line built on top of Portsdown Hill. This was rejected by a committee chaired by the Duke of Cambridge, Commander-in-Chief of the army, who whilst agreeing that a fort of Portdown Hill was required, considered that the Hilsea Lines should be retained as a physical barrier to the occupation of Portsea Island from the landward side. In the meantime, the launch of the French ironclad ‘Gloire’ and the strengthening of their navy caused a new invasion scare and obliged the government to appoint a Royal Commission to review the defence of the country and, in particular, that of the naval dockyards. This resulted in the construction of some 70 forts and batteries in England, including the Spithead and Portdown forts, which collectively became known colloquially as 'Palmerston's Follies' after Lord Palmerston (1784-1865), who was Prime Minister at the time.

The commissioners had supported the retention of the Hilsea Lines, but made a number of amendments to Jervois' original plans. The island flanking works and two rear forts were omitted and the west of the lines, which had extended southward opposite Horsea Island, truncated. The outworks on the north bank of Port Creek were also never built. The initial construction contract had been completed by March 1861 but, partly due to problems with subsequent contractors the works were not completed until 1871. However, in line with the Royal Commission forts, the Lines did not receive their armament until 1886 by which time they were already considered largely obsolete. The original armament had been intended to comprise 168, 32lb smooth-bore muzzle-loading (SBML) guns on the terreplein and 50, 64lb SBML guns in the casemates. This was overtaken by the advances in artillery technology and they were probably never installed. In 1886 the Lines were equipped with a mix of three 7-inch rifled muzzle-loading (RML) and three 7-inch Armstrong rifled breech-loading (RBL) guns, on ‘pop-up’ counterweighted Mark II Moncrieff mountings in newly installed concrete gun pits on the bastions. Additionally, twenty 7-inch RBLs were mounted in the casemates along with moveable armament of eight 40lb RMLs and four 6.6-inch howitzers on travelling carriages. All armament was removed in 1903. During the First World War a small number of anti-aircraft and other guns were mounted on the terreplein.

In 1919 the main gateway of the Lines, consisting of two vehicular and a pedestrian tunnel, was demolished to allow for road widening, followed in 1932 by a bus depot resulting in the demolition of the battery to the west of the gate. The West Demi-Bastion was converted into changing rooms for Portsmouth Grammar School. Also in 1932, the East Demi-Bastion, East Curtain and part of the East Bastion (including two of the six Moncrieff gun pits) were demolished to make way for Portsmouth aerodrome. In 1933 the western end of the moat was turned into a boating lake and lido.

During the Second World War a number of obsolete anti-aircraft guns were installed and the Lines were manned by the Home Guard. Some of the magazines were used as air-raid shelters and at least one pillbox was built on the ramparts. In 1940 a cutting was made through the Lines to access a new causeway across the Port Creek at the eastern corner of the West Bastion.

In 1986 the council purchased the remainder of the Lines and laid out parkland and walks on much of the north side of the lines. Most of the ramparts, especially at their eastern end, are, at the time of writing (2020), heavily overgrown with largely impassable footpaths (except at the west end of the Lines). A number of the casemates are in commercial use and the Portsmouth Grammar School owns the West Demi-Bastion (Bastion One) with the land behind used as their playing fields.



The Hilsea Lines consist of a linear fortification stretching across the northern end of Portsea Island. It consists of bastions linked by adjoining stretches of curtain wall. From the west, the bastions comprise the West Demi-Bastion, West Bastion, Central Bastion and the western part of the East Bastion. The rest of the East Bastion and the East Demi-Bastion were demolished in 1932. The casemates within these bastions are, somewhat confusingly, designated numerically as ‘Bastions’ rather than casemates. They comprise Bastion 1 (West-Demi Bastion), Bastion 2 (west casemate of the West Bastion – now lost), Bastion 3 (east casemate of the West Bastion), Bastion 4 (west casemate of the Central Bastion), Bastion 5 (east casemate of the Central Bastion) and Bastion 6 (west casemate of the East Bastion). The Lines are pierced by the London Road, where the eastern part of the West Curtain, gateway and part of the west flank of the West Bastion were demolished between 1919 and 1932, a northern pedestrian extension of Perrone Road at a cutting in the eastern corner of the West Bastion dating from 1940, and by the railway line which passes through the Lines via a tunnel through the East Centre Curtain. There is also a sally port in the East Centre Curtain. To the north, the Lines are protected by a water-filled ditch varying in width between 25m and 30m. Beyond this the channel of the Port Creek offered additional protection but this is not included in the scheduled area. The scheduled area is divided into four constituent parts by the bisections noted above. The sections comprise:

* the west section comprising the West Demi-Bastion (Bastion1) and the western part of the West Curtain (around 500m in length – measurements taken along the top of the ramparts where applicable);

* the west central section comprising the salient faces of the West Bastion and a ramp at the south-west corner which is all that is left of the western flank of the bastion (around 340m in length);

* the east central section comprising the eastern flank of the West Bastion (Bastion 3), West Centre Curtain and the Central Bastion (Bastions 4 and 5) (around 900m in length);

* the east section comprising the East Centre Curtain and the western flank (Bastion 6) and north-western face of the East Bastion (around 470 m in length).

The scheduled areas include the associated stretch of ditch on the north side of the ramparts.

The ramparts are constructed of chalk and earth with casemated, brick-built, batteries in the flanks of the bastions. The original profile of the ramparts consisting of terreplein, fire step and embrasured parapet, has been lost to erosion and the ramparts now take the form of a tree-covered earth mound. The terreplein was split into fire bays by transverse earth banks known as traverses. The casemates are at ground level below the ramparts and are intended to provide flanking fire along the ditch.


There are five surviving casemates (of the original six, that in the west flank of the West Bastion having been demolished) sited in the flanks of the bastions (the West Demi-Bastion only has casemates in its east flank). These are of a standard design and consist of varying numbers of gun batteries in linked brick-vaulted gun casemates to the front (north) and separate casemates at a slightly lower level to the rear for troop accommodation. At either end of the gun casemates are expense magazines and beyond these separate magazine and shell and artillery stores. The gun casemates have embrasures framed by granite blocks with circular ventilation openings above. The embrasures had iron doors, hung on cast-iron pintles, some of which survive. There are also vertical ventilation flues in the roof of each casemate with cast-iron vents. Between the front faces of the embrasures are earth mounds (merlons) which were intended to provide protection to the brickwork of the casemates from enemy fire.

The rear faces of the casemates present a series of arches capped with stone copings. Those to the barrack accommodation have five panels of multi-pane glazing with stone sills and a central plank door, beneath brick relieving arches. At either end are separate arched entrances with plank doors. Located at the flanks of the bastions were artillery ramps giving access to the terreplein. A number of these survive.

In addition to the gun positions in the casemates, there are four (of six) remaining semi-circular concrete gun pits dating from 1886 sited on the terreplein at the salients of the bastions. The gun pits have a semi-circular apron at the front to deflect incoming fire and a gun pit with a concave front and a magazine and shell store in the arms of the pit. The concrete has a masonry effect finish. Some of the concrete floors of the gun pits have the metal fixing plates and/or inset metal guide rails for the Moncrieff gun carriages.

Beneath the rear of the terreplein are a series of earth covered, single-chamber, vaulted brick expense magazines, reached via shallow flights of stone steps either side of the arched entrance. They are present both in the bastions and the curtain walls. Around 34 of these still survive, broadly conforming to a spacing of one expense magazine to each two traverses. Within the bastions, beneath the ramparts, were larger magazines known as area magazines. There were originally nine of these with the one at the salient of the East Bastion being larger and designated as the main magazine. This and the magazine in the East Demi- Bastion have been lost. The area magazines have two compartments, a magazine and lobby. There were also eight shell stores, generally beneath the ramparts or artillery ramps in the flanks of the bastions.


This consists of the West Demi-Bastion protecting the western end of the Lines and the western section of the West Curtain, truncated by the demolition of its eastern section in the 1930s. It includes the associated section of ditch. A well-defined footpath runs along the top of the ramparts of this section of the Lines reached by flights of concrete stairs at the south-west and east ends. At the salient of the bastion is a Moncrieff gun pit. Below the gun pit is one of the original area magazines. The casemates (and a shell store) occupy the eastern flank of the bastion (Bastion 1). These have their glazing and are used as changing rooms for the grammar school playing fields to the south of the bastion (2020). Just to the west of the casemates is a modern two-storey sports pavilion. This is outside the scheduled area but a section of the building extends over a revetted vault which was originally below the, now removed, artillery ramp, This section of the pavilion falls within, but is excluded from, the scheduled area. At least five expense magazines survive on this stretch of the Lines. Along the inner west flank of the bastion, south of the area magazine, are a number of rectangular brick structures with flat concrete roofs, possibly air raid shelters and/or ammunition stores, of Second World War date. WEST CENTRAL SCHEDULED AREA

This section comprises the two salient faces of the West Bastion (with its associated section of ditch) between the London Road to the west and the cut through the Lines at the north end of Peronne Road to the east. At the south-western end of the section the scheduled area extends southwards to include the western artillery ramp which runs along the western side of Rampart Gardens. At the salient of the bastion is a Moncrieff gun pit. Below the gun pit is one of the original area magazines. There are five expense magazines on the ramparts and at the eastern end is a circular, reinforced concrete, Second World War pillbox with five embrasures.


This section comprises the east face of the West Bastion, the West Centre Curtain and the Centre Bastion (with the associated section of ditch). It is bounded by the cut through the Lines at the north end of Peronne Road to the west and the railway tunnel to the east. The section contains three sets of casemates.

At the west end, the scheduled area includes a curving artillery ramp, the casemates of the West Bastion (Bastion 3) and their parade ground. The casemates retain their glazing behind modern protective mesh screens and cast-iron rainwater goods. The casemates are in use by various businesses (2020). At the top of the ramp is an expense magazine. There are four other expense magazines along the West Curtain and at its centre is a sallyport. This consists of a narrow vaulted brick passageway, 24m long, 2m wide and 2.5m high, running beneath the ramparts. Recesses open off the tunnel on either side, providing shelter for defending troops. The arched entrance on the south side of the Lines has a broad section of concave brick retaining wall with stone capping and a metal lamp bracket over the entrance. The entrance on the north side has sloping brick wing walls with concrete capping and a pair of iron-shod wooden doors.

The casemates on the western flank of the Central Bastion (Bastion 4) have lost their glazing and the arches bricked up with replacement doors inserted. The casemates are in commercial use (2020). They retain their gun ramp to the east. The Salient angles of the Central Bastion have further expense magazines and a Moncrieff gun pit.

The casemates on the eastern flank of the bastion (Bastion 5) are in a poor condition and have been badly vandalised. This section of the scheduled area terminates at the tunnel for the railway line.


This section commences on the eastern side of the railway tunnel and includes the East Centre Curtain and the western flank and salient angle of the East Bastion (with the associated section of ditch).

The stretch of curtain directly to the east of the railway tunnel has four casemates used as barracks (ie without gun positions) as well as expense magazines on the rampart. At the eastern end of the curtain is a Second World War field gun emplacement of 1940-1941 date, comprising a rectangular, reinforced concrete gun house with, below ground, a crew room on one side and a magazine on the other.

The casemates in the western flank of the East Bastion (Bastion 6) have been restored and retain their glazing and cast-iron rainwater goods. They are in use by various businesses (2020). The artillery ramp to the east has a path up onto the ramparts. Under the ramp is an area magazine.

The western salient of the East Bastion has a number of expense magazines and a Moncrieff gun pit with both guide rails and mounting plate. From here the Lines slope down to the east, where the eastern part of the lines were demolished in 1932. This end of the scheduled area includes an area of open land to the north where the ditch has been infilled.


The following are excluded from the scheduling: all modern railings, bollards, fences or fence posts, gates and gateposts, the paved, tarmacadam or concrete surfaces of all modern pathways and steps, roadways and car parks; all fixed benches, fixed picnic tables, bins, lights and lamp posts; signage and information boards; security cameras and poles; telegraph and flag poles; drains and drain covers; wooden sheds; the modern concrete footbridge over the ditch towards its western end; steel zip-wire A-frames and fixings and pontoon fixings at the west end of the ditch; the sports pavilion at the west end of the Lines including that part on top of the revetted vault (but not the vault and revetment itself). However, the ground beneath all these features is included.


The contents of this record have been generated from a legacy data system.

Legacy System number:
PO 330
Legacy System:


Books and journals
Mitchell, Garry, Hilsea Lines and Portsbridge - Solent Papers No 4, (October 2011, updated 2020)
Saunders, A D, Fortifications of Portsmouth and the Solent, (1998)
Conservation Area No 27 - Hilsea Lines: Portsmouth City Council Conservation Area Appraisal & Management Guidelines, accessed 20 August 2020 from https://www.portsmouth.gov.uk/ext/documents-external/dev-cons-area-27-guidlines-hilsealines.pdf
Victorian Forts and Artillery - Hilsea Lines, accessed 27 May 2020 from https://www.victorianforts.co.uk/pdf/datasheets/hilsealines.pdf
Plans held at the National Archives Kew - Works 43/277-281, MPHH 1/265 and 543, WO78/490, 1464, 1584, 1637, 1743, 1747, 2854 and 3147
Pritchard Architecture, Hilsea Lines - Bastion 5: Condition Survey and Mothballing Strategy (January 2019)


This monument is scheduled under the Ancient Monuments and Archaeological Areas Act 1979 as amended as it appears to the Secretary of State to be of national importance. This entry is a copy, the original is held by the Department for Digital, Culture, Media and Sport.

End of official listing

Your Contributions

Do you know more about this entry?

The following information has been contributed by users volunteering for our Enriching The List project. For small corrections to the List Entry please see our Minor Amendments procedure.

The information and images below are the opinion of the contributor, are not part of the official entry and do not represent the official position of Historic England. We have not checked that the contributions below are factually accurate. Please see our terms and conditions. If you wish to report an issue with a contribution or have a question please email [email protected].