The ten-sided stone shell keep, 58 feet (18 m), constructed in the 13th century to replace an earlier wooden structure, closely resembled those of Tonbridge and Arundel Castles.[25] The keep enclosed a number of buildings, leaving an inner courtyard only 22 feet (7 m) across.[26] Within the keep, stairs led 20 feet (6 m) down to an underground 12 feet (3.7 m) wide stone chamber, with an Early English hexagonal vault and a 54 feet (16 m) deep well providing water in the event of siege.[27]