 The most prominent of these is the Great Gatehouse, a massive three-storey fortification, comprising two drum-shaped towers of ashlar stone; originally 79-foot (24 m) high.This was heavily influenced by the Edwardian gatehouses in North Wales, such as that at Harlech, but contains unique features, such as the frontal towers, and is considered by historians Alastair Oswald and Jeremy Ashbee to be "one of the most imposing structures in any English castle".In the 1380s this gatehouse was further strengthened with a 31-foot (9.4 m) long barbican, of which only the rubble foundations now survive, around 2-foot 4 inches (0.7 m) high.