Built on the foundations of a gateway,to a Roman Fort.The Roman Fort,was one of a chain of coastal forts,called The Forts of The Saxon Shore.There are no extant remains of the fort,today.
see http://en.wikipedia.org/wiki/St._Cedd
see http://www.roman-britain.org/places/othona.htm
see http://www.essex-family-history.co.uk/othona.htm