The 16th century blockhouse is a two-storey, D-shaped design, internally, with walls of slate rubble up to thick resting on a platform cut out of the bedrock. The ground floor originally had three semi-circular gun-ports overlooking the sea and the estuary, although one has since been blocked up. On the first floor were two more gun-ports—one now filled in—and smaller windows that could have been used for smaller gunpowder weapons. The building had a fireplace and chimney, with a small guard chamber by the entrance, and was topped by a parapet walk.