The Bishop's Tower, or Bishop's Castle, is an archaeological site in the historic city center of Hamburg, Germany, containing the oldest known remains of a stone building in the city. The site includes the foundations of a circular tower and a well, originally believed to represent the 11th-century stone residence of Archbishop Adalbrand of Bremen. Later finds, however, disproved this theory and it is now considered to be part of a 12th-century defensive structure.