Cromwell's Castle mid-17th century blockhouse and 18th century gun platform on the western coast of Castle Down, Tresco


Heritage Category:
Scheduled Monument
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Ordnance survey map of Cromwell's Castle mid-17th century blockhouse and 18th century gun platform on the western coast of Castle Down, Tresco
© 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)
National Grid Reference:
SV 88181 15972

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.

Blockhouses are small, strongly-built defensive structures, built from the late 14th to mid-17th centuries and designed to house guns and protect the gunners and ammunition from attack, often while being located in a forward or exposed position. Blockhouses vary considerably in form, construction and ground plan but were typically sited as forward defences to cover anchorages, harbours, other defences and their approaches. They comprise a single free-standing structure, usually built of stone, incorporating a gun platform. The gun platform may be situated in a tower or a bastion. Accommodation for the gunners or look-out troops was of limited extent if provided at all. Of the 27 blockhouses with extant remains recorded nationally, three are located on the Isles of Scilly, each of a different design, built during separate periods and for differing purposes, demonstrating well the diversity of this class of defensive monument. Cromwell's Castle survives well as a late example of a blockhouse and one of the relatively few masonry fortifications erected during the period of the Interregnum. The addition of the seaward gun platform in the 18th century shows clearly developing aspects of fortification techniques over that century, while causing only minor changes to the earlier structure. The relationship between this monument and King Charles' Castle on the crest of Castle Down illustrates the limitations of early artillery and the consequences on the siting of defensive structures. The surviving historical evidence surrounding the contexts in which the extant blockhouse and gun platform were built, together with the preceding 16th century blockhouse, demonstrates well the national strategic importance that was attached to the Isles of Scilly during these periods.


The monument includes a mid-17th century blockhouse, known as Cromwell's Castle, situated on a small low rocky shelf projecting from the western coast of Castle Down on Tresco in the Isles of Scilly. A raised gun platform was added to the south west side of the blockhouse during the 18th century. Historical sources indicate that this monument was built on the site of a mid-16th century blockhouse. The existing blockhouse and gun platform is a Grade II* Listed Building and a monument in the Care of the Secretary of State. The existing blockhouse was built during 1651-2 and survives as a circular gun tower, 13.45m in external diameter, built of randomly-coursed rubble. The tower is up to 15.1m high overall, rising through a basement and two floors to a paved roof that forms a gun platform surrounded by walling pierced by six gun ports. Below the gun platform the walls are generally 3.8m thick, resulting in internal room diameters of 5.85m. The original entrance was on the south side at first floor level; there the doorway remains unblocked and from its base, stone supports called corbels project from the outer face of the blockhouse wall. The doorway and corbels were rendered obselete by the 18th century alterations, but the original entrance arrangement can be determined from a surviving detailed plan drawn in 1753, showing the monument at an intermediate stage in the planning of the alterations. This plan depicts a short outer gallery which the corbels would have supported beside the original entrance. The gallery extended a short distance around the wall's south west face to a flight of steps which descended to ground level at the west of the building. Within the blockhouse, a flight of stone steps led down through the wall thickness from the original doorway to the first floor level, with a single, slender, moulded pillar rising from the lowest step. The original wooden floors and staircases of the blockhouse have not survived but their positions can be gauged by slots in the walls for beams and for floor joists, oriented east-west. The joist slots indicate two timber-floored rooms below the roof gun platform, with an unlit basement beneath. The basement was 2.2m high, reached by a flight of six steps from the first floor at the WSW side. The steps descend to a central semicircular rubble platform, 2.1m in diameter and 0.6m high. A low rubble wall, 0.3m wide and 0.3m high, extends NNW from the platform to the basement wall face. The platform is surrounded by an earth floor with an exposure of bedrock along its north west edge. Above the basement, the joist slots indicate the first floor room was 2.3m high, entered by the original entrance steps on the south side. The room was provided with a large fireplace on the ESE, with a simple slab-form lintel, a chamfered surround and a transverse partition slab. The room was originally lit by two small windows provided high up on the south west and north west sides, with small moulded pillars supporting the lintels where their splays meet the inner wall face. The south west window was blocked by the 18th century alterations and replaced as a window by the doorway of the redundant original entrance. The second floor room is considered to have been originally reached by ladder or wooden steps from below. This upper room has a small but elaborate fireplace on the east, with a chamfered surround below a relieving arch, also chamfered along its lower edge. The room is lit by a single window high in the north west side. The window's inner splay extends almost tangentially to the inner wall face, opening into a small chamber in the wall thickness at the base of a spiral stair which links this room with the gun platform. The room is roofed by a four-ribbed stone vault supporting the paved gun platform above. The spiral stair to the roof passes through one and a half turns in the wall thickness to emerge at a chamfered-arched doorway in a small roofed chamber at the western side of the gun platform. The stairwell is lit by a single narrow window facing WNW. The gun platform is 9m in diameter, surrounded by an outer wall 2.2m high and 2.1m thick. A small chimney rises above the outer wall on the east side, serving the fireplaces below. Immediately north of the chimney, a steep flight of steps rises up the inner face of the wall to a walkway along the top, formerly protected by a parapet. The gun platform wall is pierced by six gun ports, facing roughly north west, NNW and NNE, and SSW, SSE and south east. The gun ports are rectangular, with long outer and short inner splays where they pass through the wall thickness, and each with a small socket to each side on the inner wall face. On the outer face of the gun platform wall, a small square panel defined by a moulded surround faces the landward approach to the blockhouse from the north east. In about 1740, the defences at the blockhouse were altered and refurbished under the direction of Abraham Tovey. This involved adding a pentagonal gun platform against the south west, seaward, side of the earlier blockhouse tower. The platform has straight sides facing north west, south east and south west, orientated respectively up, down and across the channel between Tresco and Bryher. Its remaining two sides, on the NNE and south east, converge to meet the blockhouse wall. The surface of the gun platform is 3.5m high above the base of the platform's south eastern walling and was paved to support cannon. The platform interior measures up to 13m north west-south east by up to 10m north east-south west. The platform is surrounded by a low parapet wall, 0.7m high and 2.1m thick, designed for the cannon to fire over, rather than through, contrasting with the earlier arrangement on the adjoining tower. With the addition of the platform, alterations were made to the means of entry. The plan of 1753 depicts the original blockhouse entrance still in place, as noted above, while the platform was reached by an angled flight of steps against the platform's south east wall, leading to an entrance way through that wall. That plan also shows the west side of the blockhouse's first floor room partitioned to form an ammunition room. The surviving remains are a modified version of those arrangements. A straight flight of stone steps rises to an arched doorway in the platform's south east wall. Passage through the platform wall, thickened at this point, is provided by an `S'-shaped flight of steps emerging at a small covered chamber on the gun platform surface. The original entrance to the earlier blockhouse tower was abandoned and its access gallery removed, to be replaced by a straight entrance passage cut through the WSW wall of the tower directly from the platform surface. Further modifications are also evident. A guard room with a roof sloping down from the tower was built between the platform's entrance chamber and the new entrance passage to the tower. This guardroom was provided with a fireplace in its south east wall, with a chimney above. On the opposite side of the new entrance to the blockhouse tower, a latrine chamber was built where the platform's NNE wall met the blockhouse wall. In addition to the surviving remains, our knowledge of this monument is amplified by surviving historical documentation. An account of the islands' fortifications dated May 1554 refers to `a blockhouse under the castle' on Tresco. The castle is an artillery castle now known as King Charles' Castle which, together with a later earthen artillery defence, is situated on the crest of Castle Down and overlooks this monument from 140m to the north east. The blockhouse and castle were part of a series of fortifications built in 1548-1554 during the reigns of Edward VI and Mary in response to a threat from the French. Although no visible remains are evident, it is considered that this earlier blockhouse was built on the site of this monument, to be replaced a century later, in 1651-2, by the circular blockhouse tower. The projecting shelf chosen for this earliest of these successive fortifications occupies a strategic position commanding the channel between Tresco and Bryher, one of the routes of entry to the heart of the Scillies archipelago and the deep water approach to New Grimsby harbour, the main anchorage on Tresco, situated 900m along the coast to the south east. Situated close to sea level, the site also facilitated the use of early cannon by removing the problem of having to fire steeply downwards, a major difficulty with the site of King Charles' Castle as a defence for the harbour. In 1651, work commenced on the present circular blockhouse soon after the Parliamentarian forces recaptured Tresco and the rest of the Scillies from the Royalist forces in April-May of that year. The stimulus for this building came from increasing tensions with the Dutch and particularly from the arrival of a Dutch fleet off the Scillies in March 1651 demanding reparations from the Royalist privateers based there. It was the threat of losing these strategic islands that prompted Parliament to send its fleet under Blake to recapture them from the Royalists. The addition of the seaward gun platform and other refurbishments at this monument begun about 1740 formed part of a much wider improvement and upgrading of the defences on the Isles of Scilly which Abraham Tovey was commissioned to undertake, partly in response to threats from Spain. The plan described above, dated 1753, shows the alterations at one stage in their planning. The monument was also visited in 1752 by the antiquary Borlase, who described the site and its armament at the time, with an engraving depicting it from the south east. Borlase noted the seaward gun platform as the `principal battery' at the site, armed with nine-pounder guns. From there he entered the blockhouse tower, whose lower room he described as a `guard room', while the tower platform was armed with small four-pounder cannon. Although he noted that the structure was `repaired in 1740', he recorded that it had no garrison and that the timber there was `already much decayed'. His published engraving of 1756 shows the access to the original blockhouse entrance already removed and the other visible details of the 18th century gun platform appear as today. All English Heritage notices, fixtures and fittings, modern surfaces and the cannon and their gun carriages and the modern sea defences are excluded from the scheduling, although the ground beneath them is included.

MAP EXTRACT The site of the monument is shown on the attached map extract.


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

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Books and journals
Borlase, W, Observations on Ancient and Present State of the Isles of Scilly, (1756)
Borlase, W, Observations on Ancient and Present State of the Isles of Scilly, (1756)
Bowley, R L, The Fortunate Islands: A History of the Isles of Scilly, (1968)
Bowley, R L, The Fortunate Islands: A History of the Isles of Scilly, (1968)
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)
Ratcliffe, J, Scilly's Archaeological Heritage, (1992)
Ratcliffe, J, Scilly's Archaeological Heritage, (1992)
Laws, P, 'Isles of Scilly Museum Publications' in The Buildings of Scilly, , Vol. 12, (1980)
Saunders, A D, 'Cornish Archaeology' in Harry's Walls, St Mary's Scilly; a new interpretation, , Vol. 1, (1962), 85-91
Saunders, A D, 'Cornish Archaeology' in Harry's Walls, St Mary's Scilly; a new interpretation, , Vol. 1, (1962), 85-91
Saunders, A D, 'Cornish Archaeology' in Harry's Walls, St Mary's Scilly; a new interpretation, , Vol. 1, (1962), 85-91
AM7 scheduling notification for SI 354: Cromwell's Castle, consulted 1993
Area & District of Isles of Scilly, DNH, List of Buildings of Special Architectural or Historic Interest, (1992)
consulted 1993, AM7 notification description for SI 354: Cromwell's Castle,
consulted 1993, DoE/HBMC, Ancient Monuments Terrier for SI 354, (1984)
consulted 1993, Parkes, C/CAU, AM 107 for Scilly SMR entry PRN 7354.01, (1988)
Title: 1:2500 Ordnance Survey Map, SV 8715 Source Date: 1980 Author: Publisher: Surveyor:
Title: 1:2500 Ordnance Survey Map; SV 8815 Source Date: 1980 Author: Publisher: Surveyor:
Title: Plan by 'CL', in HPG display in the Powder Magazine, St Mary's Source Date: 1753 Author: Publisher: Surveyor: Noted by MPPA on visit in 1993


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

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