The White Tower is the oldest structure in the Tower of London, dating from 1097, and was the tallest building in London until the early 20th century. William the Conqueror, before he entered London, formed a camp, eastward of the city, and probably on part of the ground now occupied by the Tower. Immediately after his coronation he commenced the works here. The White Tower is somewhat irregular in plan, for although it looks so square from the river its four sides are all of different lengths, and three of its corners are not right angles. The whole building is crossed from north to south by one wall, which rises from base to summit and divides it into a larger western and a smaller eastern portion. The eastern part is further sub-divided by a wall which cuts off St. John's Chapel, its crypt, and its sub-crypt. It is also home to the Horse Armoury, depicted in the photo to the right.