Thames Block, Fort Blockhouse


Heritage Category:
Listed Building
List Entry Number:
Date first listed:
Statutory Address:
Haslar Road, Gosport, Hampshire, PO12 2AB


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Statutory Address:
Haslar Road, Gosport, Hampshire, PO12 2AB

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

Gosport (District Authority)
Non Civil Parish
National Grid Reference:


Former officers’ quarters, now an officers’ mess. Built in 1845 to 1847 with a memorial dining hall added in about 1919. The range was extended to the south-west and a first floor added, as well as extensions to the front and rear, in about 1926 to 1929. Further additions were built to the rear during the Second World War and the entrance porch was extended in the late C20.

Reasons for Designation

Thames Block, built in 1845 to 1847 as casemates and officers’ quarters at the artillery fort known as Fort Blockhouse and altered in the C20 to form an officers’ mess, is listed at Grade II for the following principal reasons:

Architectural interest:

* as bombproof groin-vaulted casemates and officers’ quarters built in 1845 to 1847 to form part of the defensive enceinte of a key bastioned artillery fort that defended Portsmouth Harbour;

* as officers’ accommodation constructed at a time when permanent barracks were relatively rare.

Historic interest:

* as casemates and officers’ quarters built in 1845 to 1847 and incorporated into the enceinte of a bastioned artillery fort that was central to the defence of Portsmouth Harbour, subsequently becoming a Royal Engineers establishment for the use of fixed minefields for harbour defence in the late C19, before serving as a principal base and spiritual home of Britain’s submarine service during the C20.

Group value:

* with the scheduled artillery fort of Fort Blockhouse, and with the contemporary north-east casemates known as Arrogant block, the former guardhouse, the Submarine Memorial Chapel, the Admiralty boundary stone, the submariners’ memorial, the cannon bollard, the former gatehouse datestone and the Submarine Escape Training Tower (SETT), all of which are listed at Grade II.


Fort Blockhouse is sited on the western side of the entrance to Portsmouth Harbour at the end of a peninsula joined to Haslar by a narrow neck of land. A timber blockhouse was recorded on the site in the early C15. It stood opposite The Round Tower (NHLE 1386901) on the eastern side of the harbour entrance; a chain between the two securing the entrance against enemy ships. In about 1538 the west side was refortified with the addition of Lymden’s Bulwark but by the late C16 the fortifications were derelict. In the late C17, during the Second Dutch War (1672-1678), concerns over the vulnerability of naval dockyards to attack led to new schemes by the great fortress engineer Sir Bernard de Gomme to strengthen Portsmouth’s defences. An L-shaped battery was built on the site. A plan of 1668 shows a fort with defences facing the land and sea. In about 1709, there was major reconstruction work. The fort’s south-west defences were considerably strengthened with a redan, moat and outer work, as well as a south-east sea battery. The remaining north-west and north-east sides of the fort were enclosed by oak palisades with an angled bastion to the north. In the early C19 further work was undertaken, including the northwards extension of the western face of the west demi-bastion and extensive remodelling of the sea battery. Between 1845 and 1848, the sea battery was further strengthened and new casemates were built on the north and the eastern sides of the fort, as well as a limestone-faced north bastion. However, the advent of larger rifled cannon meant the defences became increasingly obsolescent as the C19 wore on.

In 1873 Fort Blockhouse was taken over by the Royal Engineers who were engaged in the use of fixed minefields as a means of harbour defence. A loading shed, workshops, mess room, boat and cable sheds, and a jetty were built. In 1904, the fort became a submarine base with the addition of new jetties and submariners accommodation. The hulk HMS Dolphin was also brought to the site to provide further accommodation and in 1912 gave its name to the newly established independent command. During the First World War, the base was the Royal Navy’s principal submarine depot and a memorial chapel was erected in 1917. A mock submarine control room known as an ‘attack teacher', was also erected along the northern curtain wall; only traces in the brickwork now remain. After the war, the outer C18 defence works were reduced, the moat partly infilled, and several buildings constructed within the fort interior, outside the main gate and surrounding it. In 1935 to 1937, a new headquarters was built for Rear Admiral, Submarines (commander of the service) with an operations room and communications facilities. At the outbreak of the Second World War, HMS Dolphin was home to 5th Submarine Flotilla. Operations were conducted in the Channel and the Bay of Biscay, and in June 1944 X-craft from HMS Dolphin were used to mark the D-Day landing beaches. After the conflict HMS Dolphin resumed its role as the main base for the submarine service, rising in prominence in the mid-1960s when the service provided the country’s nuclear deterrent. The base expanded with many new buildings on the land to the south-west of the peninsula. However, the end of the Cold War in 1991 led to a major reappraisal of defence requirements, and HMS Dolphin closed in 1998. The Submarine School was transferred to HMS Raleigh at Torpoint, Cornwall, and the Defence Medical College was established at Fort Blockhouse; the base being occupied by 33 Field Hospital. In 2016 the government confirmed that Fort Blockhouse was to close; the estimated date for disposal is 2022.

Thames Block was originally built in 1845 to 1847 as eight bombproof brick casemates, providing officers’ quarters on the north-west side of the fort; these casemates form the core of the current building. The casemates were glazed on the courtyard side and entered up steps through a single door protected by a wooden porch. Chimneys projected through their roofs. At the centre of the range a groin-vaulted passageway gave access to the exterior and this continues to form the north entrance to the fort. An 1850s print (Moore 2014, 13) and historic photographs show that the officers’ quarters were defensible with a raised loop-holed parapet commanding the courtyard with presumably a similar outward looking arrangement. A staircase at the west end gave access to the walkway of the northern loop-holed curtain wall that joined to the masonry tail of the west demi-bastion. To the rear of this wall were the officers’ water closets. A replaced or repositioned datestone to Thames Block records that the original building was completed in 1847.

In the early C20, after the fort became a submarine base, the officers’ quarters were progressively extended to create an officers’ mess. To the rear a large dining memorial hall was added after the First World War. Internally, it has a queen post roof, timber panelling and a series of paintings of sailing vessels, after which buildings are named on the site, by the renowned maritime artist and Portsmouth resident William Lionel Wyllie (1851-1931). These are considered to have been especially commissioned for this room in memory of submariners who died during the First World War (Gosport Borough Council 2007). An enclosed corridor was also subsequently added to the rear of the range as well as further quarters at the western end of the rear elevation. In the late 1920s a single-storey extension was built along the western side of the front elevation to create an entrance and ward rooms. A second storey was added to the main range, which was also extended to the south-west in about 1926 to 1929. A cast-iron fireback dated 1929 in the entrance lobby of the block probably indicates the date of completion of these works. Further additions, including kitchens and service rooms, were added to the rear in about 1926 to 1929 and during the Second World War. The 1920s and later additions are largely built in dark-red stock brick contrasting with the bright red brick used in constructing the original casemates and the 1919 dining hall. The entrance porch was extended in the late C20.


Former officers’ quarters, now an officers’ mess. Built in 1845 to 1847 with a memorial dining hall added after the First World War. The range was extended to the south-west and a first floor added, as well as extensions to the front and rear, in about 1926 to 1929. Further additions were built to the rear during the Second World War and the entrance porch was extended in the late C20. The rear mid-C20 infill ranges and rear officers’ accommodation block, are of lesser interest.

MATERIALS: originally constructed of red brick laid to Flemish bond with later additions in dark-red stock brick laid in stretcher bond. Stone and concrete dressings and slate-covered roofs.

PLAN: the core of the main range is formed of eight groin-vaulted casemates of 1845 to 1847 at ground-floor level; four to either side of a groin-vaulted passageway serving as the north entrance to the artillery fort. The main range was extended to the south-west and a central entrance hall and staircase inserted into one of the casemates in 1926 to 1929; this leads to a second storey of officer’s quarters added at that time. A single-storey extension was also added to the western end of the front elevation and three casemates opened up internally to provide a mess room. Adjacent to this is a meeting room. The four casemates at the east end of the range retain their dividing walls and plan forms but now provide offices. Extending to the rear of the main range are several later additions: the dining hall, kitchens, service rooms, and additional officers’ quarters.

EXTERIOR: the former casemates and officers’ quarters are situated on the north-west side of the bastioned artillery fort of Fort Blockhouse. The building is orientated north-east to south-west with the south-east elevation forming the main façade, facing the interior of the fort. The main range is 15 bays long and three bays wide. The ground floor of the front elevation broadly comprises, from left to right: nine bays of sash windows to the single-storey extension added in the late 1920s under a flat roof; a late C20 brick entrance porch; a groin-vaulted passageway through the range, serving as the north entrance to the fort; and four blocked round-headed openings of the original casemates. The single-storey extension appears to have been built in two phases; there is a straight-joint in the brickwork separating the first three bays at the western end from the following six bays to the east. The first three bays have a brick plinth beneath tripartite sashes and pivoted windows, each under a segmental-headed arch with red brick voussoirs and concrete keystones. Above the windows is a concrete string course and then a brick parapet with a concrete coping. The following six bays are similar in design with a brick plinth, six-over-six sashes with red brick voussoirs and concrete keystones, and a brick parapet with a concrete coping. The adjacent late C20 entrance porch has a projecting entrance containing glazed steel double-doors under a segmental-headed opening, and a parapet matching the adjoining extension. Further to the east is the vaulted passageway and then the four casemates. The first of the casemates has a timber porch and the first three all have doorways approached by brick and concrete steps with handrails. They have six-panelled timber doors with transom lights flanked by six-over-six sash windows. The final casemate at the eastern end of the range has a tripartite segmental-headed window but no door opening. The first floor of this elevation has 21 bays, largely of six-over-six sash windows with red brick voussoirs and concrete keystones. However, the window over the north entrance bay has paired four-over-four sashes flanked by brick pilasters containing narrow blocked loopholes for architectural effect. Beneath it is a datestone inscribed with the initials V R (Victoria Regina) and date 1847. Surmounting the elevation is a deep dentilled eaves cornice, a hipped slate-covered roof, and corbelled brick chimneys largely positioned off-centre towards the rear.

The south-west elevation adjoins the fort curtain wall. It is an irregular composition with one and two-storey additions extending from the rear of the main range; these have timber doors and segmental-headed sash windows. The rear elevation comprises from left to right at ground-floor level: a flat-roofed enclosed corridor; the gabled single-storey dining hall flanked by two gabled cross-wings to the main range; one and two-storey infill ranges added during the Second World War; and then a two-storey officers’ accommodation block with a hipped slate-covered roof. The mid-C20 infill ranges and officers’ accommodation block are of lesser interest. The enclosed corridor at the east end of this elevation is nine bays long, largely containing eight-over-eight uPVC sash windows. It is entered via a half-glazed timber door approached by concrete steps with a steel handrail. There are blocked embrasures probably for musketry to the original casemates on this side of the main range. Immediately to the right is the vaulted passageway to the fort and then a two-storey cross-range with a tall canted bay window lighting the staircase of the entrance hall. Further west is the dining hall which is three bays long and three bays wide. The side elevations of the hall have paired six-over-six sashes beneath top-hung windows flanked by brick pilasters and projecting chimney breasts. The gable end has two sashes flanked by pilasters and an oeil-de-boeuf set in the gable. Next to the dining hall is another gabled cross-range and several one and two-storey extensions constructed in dark-red brick with uPVC sash windows and uPVC doorways. The officers’ accommodation block at the far west is five bays long and three bays wide. It has six-over-six uPVC sashes, a plain eaves cornice, and a tall chimney, probably for an incinerator, attached to the east angle. The first floor of the main range has several sparsely separated uPVC six-over-six sash windows. The north-east elevation adjoins the North Bastion, upon which is an enclosed corridor linking Thames Block to Clyde Block (formerly Maidstone Block) via a first floor bridge.

INTERIOR: the entrance porch leads up some stairs and through glazed timber doors into the main entrance hall. The hall is covered by the groin-vault of the original casemate and is fitted out with wainscoting. On the left is a stone fireplace. This has a cast-iron grate, fireback, and hood, as well as a moulded mantelshelf and a wooden overmantel with a moulded cornice resting on two wooden Tuscan columns. There are several wooden commemorative plaques fitted to the walls; these list the commanders and captains of Fort Blockhouse, and the commanders and surgeon captains of the Royal Haslar Hospital. A double L-staircase with a wooden balustrade leads up to the officers’ quarters on the first floor. Beneath the staircase landing is a doorway leading into the dining hall. The dining hall is open to the queen-post roof and is lit by a large leaded roof light and brass chandeliers. There is wooden panelling throughout and a wooden gallery with a balustrade at the west end supported on wooden Tuscan columns. The dining hall contains a series of fitted paintings of sailing vessels, after which buildings are named on the site, by the artist William Lionel Wyllie. These are considered to have been commissioned especially for the hall in memory of submariners who died during the First World War. A wainscoted corridor leads off left from the main entrance hall. A mess room or lounge is situated at the front of the building leading off the corridor. It is covered by the groin-vaults of three original casemates except for an additional bay under the 1920s extension. The mess room is wainscoted throughout and has brass chandeliers hanging from the ceiling. There is a stone fireplace with a Tudor arch and blind tracery. It has a metal fireback decorated with anchors, crosses, fleurs-de-lis, and other motifs. Adjacent to the mess room under the westernmost three bays of the front extension is a meeting room. This has a wooden fireplace with fluted pilasters and an overmantel, and a coffered ceiling from which are hung brass chandeliers. The four casemates at the east end of the main range are accessed externally and now provide offices. These retain the original plan forms, being divided into individual casemates, as well as fireplaces. Leading off to the rear of the main range are the kitchens, bathrooms, other service rooms, and officers’ accommodation, which are of lesser interest. The first floor of the main range contains the officers’ bedrooms and bathrooms added in 1926 to 1929.


Books and journals
Evans, D, Arming the Fleet: The Development of the Royal Ordnance Yards, (1770-1945)
Hall, K, HMS Dolphin: Gosport's Submarine Base, (2001)
Moore, D, Solent Papers No.13 - Fort Blockhouse and Fort Monckton, (2014)
Saunders, A D, Fortifications of Portsmouth and the Solent, (1998)
Williams, G, The Portsmouth Papers No.30: The Western Defences of Portsmouth Harbour 1400-1800, (1979)
Francis, P, and Crisp, G, Military Command and Control Organisation, report for English Heritage (2008)
Historic England, Screening for Potential Listing Report: Fort Blockhouse, Gosport, Hampshire (2020)
The Gosport Borough Council, Haslar Peninsula Conservation Area Appraisal (2007). Accessible online at:


This building is listed under the Planning (Listed Buildings and Conservation Areas) Act 1990 as amended for its special architectural or historic interest.

End of official listing

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