The site began life as a coastal promontory fort, a type of enclosure in which natural cliff edges do much of the defensive work, with artificial banks and ditches, known as fosses, cutting across the landward side to complete the barrier. What is striking here is the degree to which the later castle was simply dropped inside this earlier structure, making the ground beneath the famous ruin a layered thing, medieval ambition built upon something considerably more ancient.

The geometry of the original fort is still legible if you know what to look for. The inner bank survives at between 1.8 and 2.4 metres high and is 7.2 metres thick, rising only slightly above what is now called the castle green. The fosse that formed the outer defensive cut was more than six metres wide. A second fosse crosses the interior, running roughly 48 metres in from the inner bank and about 10.8 metres from the castle itself, suggesting the enclosed space was subdivided in some way. The trouble is that the castle's later history has not been kind to the earlier remains. A modern path cuts directly through what was once the outer fosse, and the garden wall of the Castle Hotel has damaged the outer ring further, so that what survives is partial, compressed between centuries of use and reuse on a headland that has always attracted occupation.