The Kajsar Tower
Visit to the tourist attraction of Sweden - the part of Visby city wall - The Kajsar Tower: the first of the full-length towers was the Kajsar Tower and its name is refers to the Swedish word for Emperor (kejsare).
Wikipedia: The tower is square shaped and the main part of the building is on the outside of the wall. It was the strongest tower in the eastern part of the wall. It may have served as the town's central weapons storage. The original round-arched entrance to the ground floor is still visible in the west facade. The tower originally had four floors and was thus higher. The top floor was most likely open on the town-side and had a gallery for archers. Two of the three upper floors can be reached via an external staircase. The tower later served as the town prison.