The powerful walls of St John's Monastery (Ágios Ioánis Theólogos) dominate the town's white houses and the island of Pátmos, as well as the surrounding archipelago. The circuit wall was built in the 11C-12C; its massive canted buttresses were added in the 17C. Beyond a dark zigzag passageway the monastery's fortress-like appearance gives way to a maze of courtyards, stairs, domes and arcades. Not to be missed: the frescoes decorating various buildings, and the musuem.