Blog – Mersea Fort: Fort Mersea: Mersea Blockhouse: East Mersea Blockhouse: Mersea Tudor Blockhouse: Cudmore Grove Blockhouse.


  • Mersea Island, Essex.
  • OSGB – TM 07195 15166.
  • Scheduled Monument.


A Henry VIII Device Fort.

The fort/blockhouse is situated at the eastern end of Mersea Island in the Colne Estuary, an important location which gave access to the city of Colchester. This site was one of three in the area which protected the approach to Colchester. The other two forts being located at St. Osyth and Brightlingsea.


Aerial Image Showing Importance of Location.


The site was triangular in plan and the sides each measured 91.4 meters in length. At each angle was a circular turret which were later filled with earth to give extra strength to the site. The blockhouse included an outer ditch and a drawbridge into the fort. It was constructed of timber and earth, not the usual stone used in such buildings.


Fort Mersea Plan.
By Hchc2009 – Own work, CC BY-SA 4.0,



The internal area of the fort housed the garrison and the munitions. Each angle turret was furnished with four guns.

It was part of Henry VIII’s Device Forts, built due to his break from the Catholic Church and the fear of attack from Europe for his actions. The overseers of the building of the blockhouses are recorded as having been Richard Cawarden and Richard Lee. The cost was quite high at £2,717, equating to around £1.2 million today. The construction must have been a challenge for the area was salt marsh land, and this could explain some of the high costs. The remains of a jetty have also been discovered nearby which may have been used in association with the fort, for example the garrison, transportation of the artillery and stores.

The garrison consisted of a Captain, a Lieutenant, 2 soldiers, a porter and 3-6 gunners. However, these numbers changed drastically in a time of conflict. It is recorded that in 1650 the site held a garrison of 50 men plus cavalry.


Aerial Image Showing Importance of Location.


The site did not have a long history. It went in and out of use a few times, and was eventually left to the elements, which has included sea erosion. When the sea wall was built it cut through most of the site. One book tells us that,

‘One source describes how the new sea wall “cut right through the ruins of the old blockhouse, obliterating the remains of the two landward towers. Today one must look very hard indeed to see where the old fort once stood.” (Fautley & Garon, 2004).

Archaeology had shown there to be wooden structures still within the interior of the site.


Here is the timeline,


1543                 Built under orders of Henry VIII as one of his Device Forts, protecting the coastline of England.

1552                 “Mersey or Mersea, at the entrance of the River Colne for the defence of the river approach to Colchester” (Tregallas, 1887).

1553                 The guns were removed from the site and it was partially demolished.

1566                 Admiralty Court was held on the site.

1586                 Surveyed: The site was abandoned.

1588                 The Blockhouse was repaired and regarrisoned due to the threat of a Spanish invasion.

1631                 The Blockhouse was repaired and regarrisoned following a threat from the Dutch Dunkirkers. Reported as being in a good sate of repair.

1646                 The garrison was disbanded.

1648                 The Blockhouse was captured by Parliament, and the ramparts were reinforced. ‘29th June: Capt. Zanchie, who took Mersea Fort, found two culverins, two sacres, and one Drake in it. (Calendar of State Papers. 1893). The garrison was commanded by Captain William Burrell, who was also in charge of the island of Mersea.

1650                 The blockhouse was reinforced with 50 soldiers and cavalry. Two more guns were added.

1651                 The site was repaired.

1655                 Royalist prisoners were held at the site. Demolition ordered by Parliament. Was not undertaken due to complaints from the locals and the landowner.

19th C                Recommissioned to protect the area during the Napoleonic Wars. A new gun battery was constructed which held 6 24 pounders.

1982                 Surveyed.

2003-2003          Excavated and Surveyed.



It may have been a small site, but it does have a good history. Time is washing away it’s memory and soon there will be no evidence of the important part it played in the defence of our small island.


Mersea Fort Close Up of Remains. Note the sea wall cutting through the landward towers.


References & Bibliography.

Andrews. W. 1892. Bygone Essex. T. Forster.

Brown. S., Barton. M., & Nicholls. R. 2011. Coastal Retreat and/or Advance Adjacent to Defences in England and Wales. Journal of Coastal Conservation, 15(4), 659-670. Retrieved February 9, 2021, from

Calendar of State Papers. 1893. Preserved in the State Paper Department of Her Majesty’s Public Record Office: Domestic series, Volume 20. Longman.

Carter. M. 1810. A Most True and Exact Relation of That as Honourable As Unfortunate Expedition of Kent, Essex, and Colchester. I. Marsden.

Coller. D. W. 1861. The People’s History of Essex: Comprising a Narrative of Public and Political Events in the County, from the Earliest Ages to the Present Time: The Hundreds and Boroughs, with Descriptive Sketches of Their Antiquities and Ruins, the Seats of the Nobility and Gentry, and an Epitome of the Parochial Charities. Meggy and Chalk.

Cromwell. T. 1825. History and Description of the Ancient Town and Borough of Colchester in Essex, Volume 1. R. Jennings.

Cutts. E. L. 1888. Colchester. Longmans, Green, and Company.

Fautley. M. P. B., & Garon. J. H. 2004. Essex Coastline: Then and Now. Matthew Fautley.

Fisher. S. 2019. Coastal Britain: England and Wales: Celebrating the History, Heritage and Wildlife of Britain’s Shores. Bloomsbury Publishing.

Gardiner. S. R. 1894. 1647-1649. Longmans, Green.

Harrington. P. 2013. The Castles of Henry VIII. Bloomsbury Publishing.

Laver. H. 1889. Roman Roads Near to and Radiating from Colchester. Transactions of the Essex Archaeological Society, Vol. 3, pp. 123-135.

Lyndon. B. 1981. The Parliament’s Army in Essex, 1648: A military community’s association with county society during the Second Civil War. Journal of the Society for Army Historical Research, 59 (239), 140-160. Retrieved February 9, 2021, from

Lyndon. B. 1986. Essex and the King’s cause in 1648. The Historical Journal, 29(1), 17-39. Retrieved February 9, 2021, from

Morant. P. 1789. The History and Antiquities of Colchester, in the County of Essex: Containing a General Account of the Place. J. Fenn.

Murphy. P. 2011. The English Coast: A History and a Prospect. A&C Black.

Smallwood. T. 2010. The Date of the Gough Map. Imago Mundi, 62(1), 3-29. Retrieved February 9, 2021, from

Tregallas. W. H. 1887. Historical Sketch of the Permanent Coastal Defence of England. Professional Papers by the Corps of Royal Engineers … Royal Engineers Institute: Occasional papers, Volume 12. Royal Engineer Institute, pp. 67-86.


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