The Curfew Tower is a great D-shaped tower begun in the 1220s by Henry III to command the north-west angle of the lower ward defences. Beneath it a sally port was also created, a discrete entrance intended to allow the defenders of the castle to make surprise attacks in the event of a siege. In 1477 Edward IV granted the tower to the college as a belfry and a great timber frame was erected within it to house the bells and clock mechanism. This whole arrangement was evidently intended to be temporary because Edward IV also planned a great central tower over the new chapel - then under construction - to serve the same purpose. But the chapel tower was never built and the college bells and clock have hung here ever since. Astonishingly, the medieval timber framing of the belfry remains intact but it is completely concealed beneath 19th-century alterations: in 1863 the whole tower was refaced and heightened to receive its present, conical roof. Curiously, these changes were probably undertaken at the suggestion of Napoleon III.