priority for new investment, but the town of Harwich was reinforced in 1539 and further new fortifications, including Mersea Fort, were built along the coast in 1543, under the direction of Richard Lee and Richard Cawarden, at an estimated total cost of £2,717. The fort was positioned on salt marshland on the south side of the River Colne, and would have protected the entrance to the estuary, working together with the new blockhouses built at St Osyth and Brightlingsea. Mersea was triangular, with earthwork walls between approximately long, further protected by "maunds", round baskets filled with earth, and a defensive ditch with a drawbridge. On each of the three corners was a circular bastion housing up to four guns. Buildings in the courtyard would have housed the garrison, which comprised a captain, a lieutenant, two soldiers, a porter and between three and six artillery gunners. There was a nearby jetty, probably for use by the fort. In 1552 the fort was decommissioned, but was subsequently recommissioned by Mary I. In addition to the fort's military role, it hosted a court to oversee the oysterfields along the East Mersea coast, which were particularly lucrative but at risk of overfishing. In 1566, Colchester introduced new rules to govern the oyster trade, which were enforced by an admiralty court, held when necessary in Mersea Fort; the location was chosen so as to demonstrate the town's authority all the way up to the top of the estuary. A survey in 1586 showed the fort had been abandoned by the military once again: the defences were in a poor state, the guns' barrels were clogged with dirt and the site had been occupied by an elderly woman. It was brought back into use in 1588 and 1631 to defend against first the Spanish Armada and then the Dunkirker privateers. English Civil War and Interregnum During the First English Civil War between the supporters of King Charles I and Parliament, Mersea Fort saw no action; after the victory of Parliament in 1646, the fort's garrison was demobilised. Conflict flared again in 1648 with the outbreak of the Second English Civil War, and Colchester was taken by the Royalists. The town was immediately besieged by General Thomas Fairfax, who quickly seized Mersea Fort, which controlled the supply route by river into Colchester, before the Royalists could do so. The fort had contained five pieces of artillery: two