 The 16th century blockhouse is a two @-@ storey , D @-@ shaped design , 5 by 4 @.@ 4 metres ( 16 by 14 ft ) internally , with walls of slate rubble up to 1 @.@ 35 metres ( 4 ft 5 in ) 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 . 
