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

A Scheduled Monument in Tresco, Isles of Scilly

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Latitude: 49.9623 / 49°57'44"N

Longitude: -6.3495 / 6°20'58"W

OS Eastings: 88180.021513

OS Northings: 15968.460905

OS Grid: SV881159

Mapcode National: GBR BXQS.6FY

Mapcode Global: VGYBX.W6NS

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

Scheduled Date: 9 October 1981

Last Amended: 24 July 1998

Source: Historic England

Source ID: 1013275

English Heritage Legacy ID: 15404

County: Isles of Scilly

Civil Parish: Tresco

Traditional County: Cornwall

Lieutenancy Area (Ceremonial County): Cornwall

Church of England Parish: Isles of Scilly

Church of England Diocese: Truro


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
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
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
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.

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

Source: Historic England

Reasons for Scheduling

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'
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.

Source: Historic England


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

Title: 1:2500 Ordnance Survey Map; SV 8815
Source Date: 1980

Title: Plan by 'CL', in HPG display in the Powder Magazine, St Mary's
Source Date: 1753
Noted by MPPA on visit in 1993

Source: Historic England

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