About
The Tudor blockhouse on Mersea Island is a rare and significant example of coastal fortifications from the reign of Henry VIII, uniquely marked by its surviving earthworks. Built in 1543, this defensive structure was commissioned as part of a network of fortifications designed to protect England’s vulnerable coastline from invasion, particularly targeting the entrance to the Colne estuary, a key strategic point.
Blockhouses like this one were typically constructed to house a small artillery garrison and to protect crucial locations such as river mouths, harbor entrances, or anchorages. Designed with stone and earthen components, they usually consisted of towers and bastions or gun platforms. The Mersea blockhouse features a triangular layout with semicircular gun emplacements at each corner, designed to hold a total of 12 guns. These gun platforms would have provided a commanding view of the surrounding estuary, offering a formidable defense against any approaching enemy ships.
The blockhouse was one of three such structures in the area, working in conjunction with blockhouses at St. Osyth and Brightlingsea. Although the other blockhouses no longer survive, the Mersea blockhouse remains the only visible example of Tudor earthworks in Essex, making it an invaluable piece of the region’s military history.
Though its original cannons were dismounted by 1553, the blockhouse continued to serve an important role in England’s defense. It was repaired and refortified during the Armada crisis of 1588, when England feared an invasion by the Spanish fleet, and again in 1631 during the threat posed by Dunkirkers, pirate-like privateers who raided the Essex coast. The blockhouse even played a role in the English Civil War in 1648, when it was captured by Parliamentarian forces, allowing them to gain control of the Colne during the Siege of Colchester.
Despite its military importance, the blockhouse faced demolition orders in 1655 under Oliver Cromwell, but the landowner at the time refused to allow its destruction, ensuring its partial survival to the present day.
Know Before You Go
The blockhouse is located in a remote area of East Mersea on the salt marshes, about 300 meters south of the nearest car park. Be prepared for a walk to reach the site.
Although much of the original stone structure is gone, the triangular earthwork foundations are still visible, offering a rare glimpse into Tudor military design. Bring a map or GPS to help locate the earthworks.
The terrain around the blockhouse can be uneven and muddy, particularly at high tide or after rain, so wear appropriate footwear for walking on marshy ground.
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Published
June 19, 2026
