[1] The fort of Othona is in a typical late 3rd-century style, and was possibly constructed during or shortly prior to the Carausian Revolt, making it contemporary with the forts at Dubris (Dover), Portus Lemanis (Lympne) and Gariannonum (Burgh Castle).

[3] Othona's location at the edge of the Dengie Peninsula was ideal for control of the estuaries of the rivers Blackwater and Colne, the latter leading to the important city of Camulodunum (now Colchester).

The stone rampart was 4.2 metres thick, indicating a tall superstructure, and enclosed over 2 hectares (4.9 acres).

Although some of the Roman building material was reused in the 7th-century Chapel of St Peter-on-the-Wall, enough of the rampart survived until the 17th century, when it was described by the local historian Philemon Holland as a "huge ruin".

The chapel belongs to Chelmsford Cathedral and is in regular use for Church of England services and by the Othona Community.