Granus Tower, Medieval tower in Aachen, Germany

The Granus Tower is a square structure of rough-hewn stone, 8.85 metres on each side and about 20 metres high, with three storeys each containing a vaulted room. Stairs run between the inner rooms and the outer masonry. A water-flushed latrine was installed in the ground-floor vault. Since 1330 it has formed the east tower of Aachen Town Hall.

Built from quarry stone and completed in 788 as part of Charlemagne's palace complex, the tower was taken over by the city of Aachen in the 14th century along with the decaying palace remains. When the town hall rose on the foundations of the Aula regia, the tower was heightened by about 14 metres in 1330 and became its east tower. In the 16th century it housed the city's charter archive, whose documents survived the 1656 fire that destroyed the rest of the records.

The name is traced by current research to Grannus, a Celtic healing god also recalled in the Roman spa name Aquae Granni. From the 16th century the tower held Aachen's charter archive; during the 1656 city fire the charters stored here survived while records kept elsewhere burned. A cross with lanterns was mounted on the tower on 1 September 1754.

The tower forms the eastern tower of Aachen Town Hall on the Markt hill; the town hall interior can be visited on guided tours, but the tower itself is not open for climbing. The tower leans about 25 centimetres to the south and east, which was stabilised in the 1960s with twelve anchor rods. Its lower rooms are reached by stairs set between the inner rooms and the outer masonry.

The tower's purpose remains unresolved: it could not be heated, had no sanitary facilities on the upper floors, and its small windows were covered with greased animal skins in iron frames against rain and snow. Researchers now consider it a stairwell giving access to the upper floor of the palace hall's porch and to the late-antique ring wall around the market hill.