northern France to provide details in the Tower's facing, although little of the original material survives as it was replaced with Portland stone in the 17th and 18th centuries. Reigate stone was also used as ashlar and for carved details. Its location, in the lower courses of the building and at higher levels corresponding to a building break, suggest it was readily available and may have been used when access to Caen stone was restricted. As most of the Tower's windows were enlarged in the 18th century, only two original – albeit restored – examples remain, in the south wall at the gallery level. The tower was terraced into the side of a mound, so the northern side of the basement is partially below ground level. As was typical of most keeps, the bottom floor was an undercroft used for storage. One of the rooms contained a well. Although the layout has remained the same since the tower's construction, the interior of the basement dates mostly from the 18th century when the floor was lowered and the pre-existing timber vaults were replaced with brick counterparts. The basement is lit through small slits. The entrance floor was probably intended for the use of the Constable of the Tower, Lieutenant of the Tower of London and other important officials. The south entrance was blocked during the 17th century, and not reopened until 1973. Those heading to the upper floor had to pass through a smaller chamber to the east, also connected to the entrance floor. The crypt of St John's Chapel occupied the south-east corner and was accessible only from the eastern chamber. There is a recess in the north wall of the crypt; according to Geoffrey Parnell, Keeper of the Tower History at the Royal Armouries, "the windowless form and restricted access, suggest that it was designed as a strong-room for safekeeping of royal treasures and important documents". The upper floor contained a grand hall in the west and residential chamber in the eastboth originally open to the roof and surrounded by a gallery built into the walland St John's Chapel in the south-east. The top floor was added in the 15th century, along with the present roof. St John's Chapel was not part of the White Tower's original design, as the apsidal projection was built after the basement walls. Due to changes in function and design since the tower's construction,