The Tower of London dungeon, known as the White Tower, is the most characteristic part of the building. The 31m-tall stone walls form an irregular quad with a round tower and three square towers at the corners. They house the Royal Armouries collection and St John's Chapel, which is 17m long and has two levels. The Beauchamp Tower, built in the 13C, was a prison (prisoner graffiti can be seen in the main room).