The Gunpowder Tower
Visit to the tourist attraction of Sweden - the part of Visby city wall - The Gunpowder Tower: is the oldest tower in the city wall of Visby and was built as a defensive tower to protect the medieval harbour.
Wikipedia: The tower was built in 1160–61, in connection with the conclusion of a trade agreement with the Duke of Saxony as a free-standing defensive tower. From the top of the tower, crossbowmen could shoot straight into approaching ships. The tower had no gate, only a small hatch about 10 metres (33 ft) up the wall, accessible via a removable ladder. This design made it more difficult for an aggressor to gain access to the tower.