I believe this may be the remains of an ancient building and that the structure may relate to the castle referred to in the Chronicle of Battle Abbey when it stated The castle then stood below the cliff, on ground since overflowed by the sea . The ground in question has undoubtedly at some stage been flooded by the sea and it is located immediately below a headland that also was a cliff many years ago. The statement in the Chronicle has always baffled historians. Here there appears to be the potential for an answer that interfaces with all the other elements of the puzzle. I am certain that they relate to the port and are of great antiquity.
Further investigation is essential in order for others to provide the necessary explanation.