Othona was built overlooking the Blackwater estuary at Bradwell. A unit (numerus) of Fortenses is listed at Othona in the Notitia Dignitatum. The Old English Bradewealla is the name of the Blackwater. Bradwell near the Sea is Effecestra in the Domesday Book. This is Bede's Ythancaestir.

Bede also states that Ythancaestir lay 'on the banks of the Pente' using what may have been the British name for the Blackwater. The name also occurs twice in the poem on the battle of Maldon in the forms 'Panta' and 'Pantan'.

According to Ekwall, Ythan | ceastir is not linked to the name Othona but the similarity of the first element is disturbing. Bishop Cedd's church, built in 654 for Saeberht king of the East Saxons, reuses stone from the fort.