Facing the cathedral across Exchequer Gate, this castle was built by William the Conqueror in 1068, incorporating the remains of Roman walls. The castle was used as a debtor's prison from 1787 to 1878. In the chapel you can see cagelike stalls where convicts heard sermons; they were designed this way so inmates couldn't tell who their fellow prisoners were, thus supposedly preserving a modicum of dignity. The castle's star exhibit is an original copy of Magna Carta,
signed by King John in 1215. This is one of only four surviving copies, and one of few ever to have left the country—it was secretly moved to Fort Knox for safekeeping during World War II. At this writing the castle was undergoing a major renovation project for the 800th anniversary of the Magna Carta signing in 2015; new attractions will include an underground exhibition center devoted to the document, a high-tech interpretive center, and an historic walk starting from the castle and covering the entire route of the old Roman walls.