The Mill Tower
Visit to the tourist attraction of Sweden - the part of Visby city wall - The Mill Tower: the tower is semi-circular, which is unusual for Visby but common in the Baltic states and the tower was built as part of the first, lower wall and was kept intact when the height of the wall was later increased.
Wikipedia: Inside the tower, openings to a former archer's gallery are still visible. The tower originally had five floors and an upper part with battlements and the side of the tower facing the town was completely open, and a wall covering that side was added later. On the tower façade, two stones that have been shattered by cannonballs are still visible.