Curfew Tower is the largest and most imposing of the three towers on the west face of Windsor Castle. Curfew Tower is at the opposite end of the curtain wall from Salisbury Tower and near Marbeck's. Curfew Tower's strategic location, overlooking the River Thames and Eton, led to the casting of a special gun, made in 1544, that was trained on the bridge over the river. Built by the House of Plantagenet during the reign of Henry III, the tower was begun in 1227 and completed in 1230 with walls 14 feet thick in places. The gabled roof was added in 1863 and was inspired by the Tower at Carcassonne in southwest France. The dungeon, in the early English Gothic style, was a storeroom for weapons. During the English Civil War (1642-1649) it was almost certainly a dungeon. Originally the ground floor of the Tower served as the Guard's living and working area. Just inside the main entrance to the Tower is a trap door that opens into a passage built through the castle wall leading to the base of the hill. This passage enabled defenders to "sally out" at besiegers through a gate known as the Sallyport.

The clock, at the top of the Tower, was made by John Davis from 1660-1689 and still contains the original bushes, bearings and counterweight system. Until Cromwell's time the Clock Tower had no floor so the guards had to perch on ledges during their watch.