The sixteenth-century Great Sea Gate, which straddles Pikk at its far end, is flanked by two towers. The larger of these, Fat Margaret Tower, has walls four metres thick and now houses the Estonian Maritime Museum, a surprisingly entertaining four floors of nautical instruments, scale models of ships and antique diving equipment. Down Gümnaasiumi, is one of the longest extant sections of Tallinn’s medieval city wall. The 4km of walls that surrounded the Old Town were mostly constructed during the fourteenth century. Today, 1.85km of it still stands, along with twenty of the original 46 towers. You can enter three of the oldest towers, Nunne, Kuldjala and Sauna, from Gümnaasiumi 3.