This natural cavern, unparalleled in other British castles, is below the Northern and Norman Halls, formed in the limestone rock on which Pembroke Castle is built. Access to and from the castle is gained by a circular staircase built half in the rock and half in an exterior turret.
In shape it is a roughly circular vault measuring about 80 feet from north to south, and 60 feet from east to west with a lofty roof. It was probably used as a storehouse and boathouse and was fairly dry when the buildings over it were roofed. In front of the arched entrance, a canal or ditch ran down to the river; this ditch was filled in when forming the path around the castle. It was probably used by small ships so that they could load and discharge cargo directly from the cave. The sally port overlooking the river is now made secure with an iron grille.
Warning! Outside at the north-east angle, in a shoulder of the wall, is an arch which is the outlet of the latrine in the Northern Hall above!
Reprinted with cringing permission from Caves of West Wales by Tony Oldham (2003).