The Gunpowder Tower (Kruttornet in Swedish) is the oldest tower in the Visby City Wall. It was built to protect the medieval harbor which was located in the part of Visby now known as Almedalen park. It was built in the 12th century, in connection with the conclusion of a trade agreement with the Duke of Saxony as a free-standing defensive tower. It is believed that it was originally located on a rocky reef surrounded by water. An identical tower stood on the opposite side of the harbor entrance, but it did not survive. The harbor has also vanished. From the top of the tower, crossbowmen could shoot straight into approaching ships. The tower had no gate, only a small hatch about 10 meters up the wall, accessible via a removable ladder. This design made it more difficult for an aggressor to gain access to the tower.

During the wars with Denmark in the 14th and 15th centuries, the tower served as a gunpowder store for both sides, since it was not unusual for the representatives from the town of Visby to change their allegiance.

The interior of the tower is well-preserved, with its vaulting, steps, and privy shaft. There are ancient inscriptions on doors and walls. There was no heating and the tower was never lived in, though it did serve for a time as a prison.