Harry's Walls unfinished 16th century artillery castle and adjacent prehistoric standing stone at Mount Flagon, St Mary's


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Scheduled Monument
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Ordnance survey map of Harry's Walls unfinished 16th century artillery castle and adjacent prehistoric standing stone at Mount Flagon, St Mary's
© Crown Copyright and database right 2019. All rights reserved. Ordnance Survey Licence number 100024900.
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The building or site itself may lie within the boundary of more than one authority.

Isles of Scilly (Unitary Authority)
St. Mary's
National Grid Reference:
SV 90963 10913

Reasons for Designation

The Isles of Scilly, the westernmost of the granite masses of south west England, contain a remarkable abundance and variety of archaeological remains from over 4000 years of human activity. The remote physical setting of the islands, over 40km beyond the mainland in the approaches to the English Channel, has lent a distinctive character to those remains, producing many unusual features important for our broader understanding of the social development of early communities. Throughout the human occupation there has been a gradual submergence of the islands' land area, providing a stimulus to change in the environment and its exploitation. This process has produced evidence for responses to such change against an independent time-scale, promoting integrated studies of archaeological, environmental and linguistic aspects of the islands' settlement. The islands' archaeological remains demonstrate clearly the gradually expanding size and range of contacts of their communities. By the post- medieval period (from AD 1540), the islands occupied a nationally strategic location, resulting in an important concentration of defensive works reflecting the development of fortification methods and technology from the mid 16th to the 20th centuries. An important and unusual range of post- medieval monuments also reflects the islands' position as a formidable hazard for the nation's shipping in the western approaches. The exceptional preservation of the archaeological remains on the islands has long been recognised, producing an unusually full and detailed body of documentation, including several recent surveys.

Artillery castles were constructed as strong stone defensive structures designed specifically to house heavy guns. Most date from the period of Henry VIII's maritime defence programme between 1539 and 1545, though the earliest and latest examples date from 1481 and 1561 respectively. They were usually sited to protect a harbour entrance, anchorage or similar feature. These monuments represent some of the earliest structures built exclusively for the new use of artillery in warfare and can be attributed to a relatively short time span in English history. Their architecture is specific in terms of date and function and represents an important aspect of the development of defensive structures generally. Although documentary sources suggest that 36 examples originally existed, all on the east, south and south east coasts of England, only 21 survive, all examples of which are considered to be of national importance. Two of these are situated on the Isles of Scilly: Harry's Walls on St Mary's and King Charles' Castle on Tresco; both originating in the same phase of fortification during the reign of King Edward VI. The islands also contain a third fortification, the Star Castle on St Mary's, which is closely related to, but slightly later than, the artillery castles. Standing stones are prehistoric ritual monuments dating to the later Neolithic and Bronze Age (c.2500-700 BC). They comprise single or paired upright slabs, ranging in height from uder 1m to over 6m where still erect. Although often conspicuously sited, many are located in relatively sheltered settings. Excavations have demonstrated sub-surface features adjacent to standing stones, including funerary cists, spreads of pebbles and various pits and hollows. In addition to having a ritual function, standing stones may have acted as markers for routeways, territorial boundaries, graves and meeting points. Standing stones are important for our understanding of ritual, territoriality and land use among prehistoric communities. Estimates suggest that about 250 standing stones are known nationally. Seven examples are known to survive on the Isles of Scilly, all of which are considered to be of national importance.

This artillery castle, known as `Harry's Walls', has survived well to the point where its initial construction was abandoned. Despite some subsequent robbing of facing stone, it displays clearly the essential details of its design. As the earliest fort in this country whose plan was wholly based on Italian-derived designs of angled bastions and orillons, this monument occupies an important place in the development of military fortifications that is reflected in its mention in national reviews of the subject. In this respect, the survival of the detailed contemporary ground plan of this artillery castle's intended form is a rare and valuable supplement to the uncompleted physical remains. This is further strengthened by the other surviving contemporary documentation giving both specific and broader historical details surrounding the commencement of this castle. The premature abandonment of this artillery castle preserves rare evidence for the manner in which such strongholds were constructed, evidence usually masked or destroyed in a completed structure. The poor siting and resulting failure to complete the artillery castle, coupled with its wealth of related historical documentation, also gives a rare insight into how fortifications were planned in this period and the degree of central control and bureaucracy involved. The standing stone in this monument has also survived well, despite its modern support cladding. Its situation on a small cairn is rare and the antiquity of this association receives unusually early confirmation in its mention by Borlase. The failure of the workers on the artillery castle to topple this standing stone for use in their structure demonstrates a respect for such antiquities that has sometimes persisted in the folklore of recent communities. The stone's survival is also considered to reflect its useful siting as a navigational aid for seamen entering St Mary's Pool, a role now superseded by the modern marker adjacent to the standing stone.


The monument includes an unfinished artillery castle, known as Harry's Walls, dating to the mid-16th century and situated on the shallow-domed summit of Mount Flagon, overlooking St Mary's Pool, on south western St Mary's in the Isles of Scilly. A prehistoric standing stone is situated adjacent to the north west side of the artillery castle. The artillery castle and the land including the standing stone are in the care of the Secretary of State. The artillery castle is defined along its south west side by a massive stone curtain wall terminating in a pointed bastion at each end, facing west and south respectively. The north west side of the artillery castle is defined by an unfinished rock-cut ditch. The curtain wall along the south west side of the castle measures 27m long along its outer face, by up to 6.6m wide, and rises up to 1.7m high externally and 1m high internally. It is faced on each side by granite blocks, generally 0.5m long by 0.2m high, with granite rubble infilling the wall core. Many of the facing blocks have been robbed in later periods for use as building material elsewhere, exposing the rubble core in the sides. A drain passes through the curtain wall near its midpoint; its inner-face opening is above the present ground surface there but indicates the intended floor level within the finished structure. At the ends of the curtain wall are large pointed bastions, both of similar design and built of faced rubble walling, similar to the curtain wall. Each bastion is defined by a massive acutely-angled wall pointing directly away from the corner of the castle, the outer faces of the point set well forward of the lines of the existing and projected curtain walls. The bastion walls average 21m long along the outer faces and 5m wide. These walls rise up to 2.3m high externally, in the castle's west bastion, though their height varies considerably to accommodate changes in the slope around the hill. Along the base of the walls, footing slabs project up to 0.6m from the faces. The angled outer walls of each bastion are joined to the ends of the curtain wall by a short linking wall. These linking walls are set slightly back from the projecting ends of the bastions' outer faces, those projections being termed `orillons'. The linking walls, surviving from 1.2m to 1.9m high, were designed to mount guns which would provide flanking fire along the outside of the curtain wall. These guns would be protected by the generally higher level of the bastions' outer walls and by their situation at the base of the recesses created by the orillons. Within each bastion is a small subtriangular internal area linked to the castle's interior by a narrow entry passage through the walls. These passage remains incomplete in the west bastion, which retains evidence for paved internal flooring. In the south bastion the entry passage measures 1.2m wide, defined on its east side by a small stub of the south east curtain wall, the only part of that wall to have been built. Apart from the stub of the south east curtain, work on the artillery castle was abandoned before the curtain walls facing north west, north east and south east, or the north and east bastions were constructed. However along the north west side, immediately beyond the projected line of the north west curtain wall, work was commenced on a broad ditch cut into the granite bedrock. The ditch, also unfinished, survives over 25m long and is generally 8m wide at ground level and 1.2m deep. Fractured outcrops of bedrock are visible along its sides and similarly irregular bedrock protrusions occur along the roughly flattened floor. The surviving south west curtain wall, the stub of the south east curtain wall and the north west ditch indicate an intended square interior for the castle measuring approximately 30m from side to side. No surface remains of internal buildings are presently visible though traces of such, parallel to the south west curtain wall, have been recorded by a previous observer. During preparation for public display, clearance of some deposits overlying the masonry at the artillery castle revealed a spherical iron shot, 8.25cm diameter, appropriate to have been used in a 16th century form of small cannon called a saker. This clearance also revealed that after abandonment the castle's site was used as a dump for rubbish, which filled the bastions and contained quantities of 17th and 18th century pottery. This clearance episode also confirmed that the extant remains of the artillery castle are incomplete because the structure was not finished, rather than as a result of stone-robbing in subsequent periods. In addition to its physical remains, our knowledge of this artillery castle is considerably amplified by reference to it in several contemporary and later documents. Of especial importance among these is an undated but broadly contemporary detailed measured plan identified as depicting the intended final form and manner of armament of this artillery castle. The dimensions given on the plan correspond closely, and often exactly, with the visible physical remains at this monument. The plan is entitled: `This fortress begonne in oure ladies ilande for the defence of the whole Isles, and not finished, the tymber work for the same alredy framid to the setting up, with a brew house and a milne lying in South Wales, redy to be conveyed to the saide Isles, when order may be given as touching the same'. The plan confirms the intended square curtain wall with acutely-pointed bastions at each corner, each with orillons to protect guns giving flanking fire along the curtain wall. A parapet is shown along the top of the bastion walls, while the curtain wall is depicted as having a stepped inner profile. Within the curtain wall is a range of internal buildings along each side, around a square central courtyard, 12.8m across. Historical research has demonstrated that this artillery castle was begun in 1551, during the reign of Edward VI, as part of a major phase of fortification of the Isles of Scilly undertaken from 1547-1554 to counter threats from France. A letter of instruction survives dated 27 May 1551 ordering John Killigrew, the paymaster of the fortification works, to `make the fort in our Ladies Isle on Scilly uppon the little hill betwixt the freshe water and St Marie Roode ...', according to a plan being sent with his son, and with supplementary instructions for a brew house and horse mill outside the castle. This location identifies the site of this monument precisely, and the latter structures conform to the intended arrangements given on the plan mentioned above; the use of timber from Wales as stated on the plan is also confirmed in documents ordering supplies for this phase of fortification. Further documents relate to the building work continuing in 1552 and 1553. In May 1554, two sakers were referred to as mounted `apon the wales of the new forte or plott to beat the harbour ... ', which has been identified with this monument. By 1554, some of the garrison on St Mary's was transferred to Tresco, reflecting a shift to that island in the focus of fortification that resulted in the completion of King Charles' Castle as the principal stronghold built in this phase. No further mention of this artillery castle is present in later accounts of the islands' defences. This artillery castle is the first fortification in this country to have been provided entirely with angled-bastions and straight orillons. This style of bastioned defence was developed in Italy and was at the forefront of military engineering design when it was used for the plan here. The failure to complete this artillery castle has been ascribed to the unsuitability of the site, whose features were clearly not understood when the plan was drawn up. The crown of Mount Flagon is too small to accommodate the size of this artillery castle as intended on its plan; the bastions needed to have been built high above the steeply sloping hill to maintain cover along the curtain wall, and this would have rendered the bastion faces both conspicuous and vulnerable to attackers. The site was also strategically poor, covering only the bay itself of St Mary's Pool but masked by surrounding promontories and headlands from covering the important approaches to the anchorage. The prehistoric standing stone is situated near the summit of the hill, 0.5m beyond the outer edge of the artillery castle's rock cut ditch, towards its northern end. The standing stone is visible as an upright granite slab, 2.75m high, whose lowermost 0.6m above the ground is embedded within a modern cladding of concreted rubble to assist in the slab's support. Where it emerges from the cladding, the slab measures 0.8m WNW-ESE by 0.3m thick, and it tapers to 0.4m WNW-ESE by 0.2m thick at the tip. The faces and edges are heavily weathered and irregular. Each principal face bears a small subtriangular pit, 2cm in diameter and located 1.5m above the ground, resulting from a relatively recent unfinished attempt to perforate the slab. The base of the slab, with the modern cladding, is situated within a slightly raised sub-circular cairn whose mound measures 7m north east-south west by 4.5m north west-south east, rising up to 0.7m high on the north west. The south east edge of this cairn is truncated by the rock-cut ditch of the artillery castle, where it can be seen to be underlain by the granite bedrock, however much of the remainder of this cairn supporting the standing stone is considered to be artificial. This standing stone was recorded by the antiquary Borlase in 1756, describing it as `on a little tumulus near Harry's Battery'. A modern navigation aid with cross-arms on a steel post and set in a concrete plinth also stands on the cairn immediately north west of the cladding around the standing stone. All English Heritage notices, fittings, stored materials, the modern navigation aid north west of the standing stone, together with its concrete base, power line and service trench, the gate, telegraph pole, guy and supply lines at the north east edge of the monument are excluded from the scheduling, although the ground beneath is included.

MAP EXTRACT The site of the monument is shown on the attached map extract. It includes a 2 metre boundary around the archaeological features, considered to be essential for the monument's support and preservation.


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

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Books and journals
Ashbee, P, Ancient Scilly, (1974)
Ashbee, P, Ancient Scilly, (1974)
Borlase, W, Observations on Ancient and Present State of the Isles of Scilly, (1756), 11
Borlase, W, Observations on Ancient and Present State of the Isles of Scilly, (1756), 16
Morgan, K O, The Oxford Illustrated History of Britain, (1984)
O'Neil, BH St J, Ancient Monuments of the Isles of Scilly, (1960)
O'Neil, BH St J, Ancient Monuments of the Isles of Scilly, (1960)
Rowse, A L, Tudor Cornwall, (1941)
Russell, V, Isles of Scilly Survey, (1980)
Saunders, A D, 'Cornish Archaeology' in Harry's Walls, St Mary's Scilly; a new interpretation, (1962), 85-91
Saunders, A D, 'Cornish Archaeology' in Harry's Walls, St Mary's Scilly; a new interpretation, (1962), 85-91
consulted 1993, Parkes, C/CAU, AM 107 for Scilly SMR entry PRN 7567, (1988)
consulted 1993, Parkes, C/CAU, AM 107 for Scilly SMR entry PRN 7568, (1988)
Deed Plan dated May 1948, DoE/HBMC, Ancient Monuments Terrier and Deed Plan for SI 351, (1984)
Deed Plan dated May 1948, DoE/HBMC, Ancient Monuments Terrier and Deed Plan for SI 351, (1984)
District of Isles of Scilly, DNH, DNH List of Buildings of Special Architectural or Hist Interest, (1992)
Title: 1:2500 Ordnance Survey Map, SV 8715 Source Date: 1980 Author: Publisher: Surveyor:


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.

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