The fort built in 1543
Henry VIII commissioned the blockhouse in 1543 to protect the entrance to the River Colne, alongside other defences at St Osyth and Brightlingsea. It was built from earth and timber rather than masonry and formed a triangular enclosure with gun positions at its three corners. Documentary records describe twelve guns and accommodation for a small garrison inside.
Historic England's scheduled-monument record locates the remains on saltmarsh between the beach and the modern sea wall.
New archaeological surveys
Historic England surveyed the surviving earthworks in late 2022 using ground survey and drone photography. Further visits in 2024 and 2025 recorded timber remains appearing on the foreshore as the coast eroded. The findings, published on 10 August 2026, include updated radiocarbon modelling of earlier discoveries. The fort had a triangular earth-and-timber plan with bastions at its three corners.
The fieldwork combined an earthwork plan with drone orthophotography and a 3D model. Repeated surveys of the foreshore were necessary because timbers thought to be related to the fort were appearing as the saltmarsh and beach eroded. Historic England’s 2026 report also revisits radiocarbon dates taken from foreshore finds sampled in 2003 and 2016.
A separate aerial and lidar study published in September 2025 traced changes in the surviving defences using historic photography and survey evidence. The 2026 study documents intermittent military use until 1655 and an unfinished scheme to reuse the site as a battery during the Napoleonic Wars.
Historic England: 2026 archaeological survey · Historic England: 2025 aerial assessment
Armada and Civil War
The guns were removed in 1553, but the fort was repaired during the Armada threat in 1588 and again in 1631 when raids from Dunkirk were feared. In 1648 Parliamentary dragoons captured it during the siege of Colchester, allowing Parliamentary vessels to move along the Colne. A demolition order issued in 1655 was not carried out after the landowner objected.
The oyster court at the Blockhouse
Colchester's Colne Oyster Fishery court once met at the East Mersea Blockhouse. A surviving minute book records that the customary court had moved to the Ship Inn in East Donyland following the Oyster Fishery Act of 1758. At its meeting on 24 July that year, Isaac Decker was appointed water serjeant and bailiff. The account records the Blockhouse as the earlier court venue, without specifying when its meetings there ended.
Colne Oyster Fishery court minute book, 1758–1763.
Visiting the site
The blockhouse survives as low earth banks on marshy ground close to the shore, roughly 80 metres along the longest side and up to 45 metres across. Parts of its banks have been lost to erosion, and there are buried remains under the saltmarsh. The scheduled site is vulnerable to further loss: stay on permitted routes, respect restrictions and do not disturb the earthworks.
The dimensions visible today should not be mistaken for the complete Tudor layout. Earlier documentary evidence described sides of about 300 feet (91 metres); much of the original enclosure has been altered or lost, and the 2026 survey suggests that part of the northern rampart may survive within the modern sea bank. The published surveys document a changing archaeological site, not a reconstructed fort or a maintained visitor attraction.