Overwintering armies needed defensible camps. Natural islands such as Thanet and the Isle of Sheppey were the first choices, but according to the Anglo-Saxon Chronicle, Vikings built forts from the late ninth century onwards:
Forts are hard to recognise and date, and names such as Danes’ Dyke (Humberside) may be misleading, as earlier features were often given a Viking attribution by later historians.
The sites are likely to have used sea, river or marsh as protection on one side, with a D-shaped enclosure such as those around coastal trading sites at Birka and Hedeby. Such a site is known from Repton and was built by the Viking army in 873-4.
An unexcavated site on Ray Island, close to Mersea Island in the Blackwater Estuary, may be a Viking site but this has not been confirmed.