The Great Tower is the dominant building on Christianso, a squat round bastion of dark Bornholm granite at the southern end of the island. It was built in the 1680s as part of the original fortifications and now serves a more peaceful purpose: a small museum on the lower floor, a working lighthouse on top.
Climb the spiral staircase for the long view across the Baltic — on a clear day you can see Bornholm to the southwest and, very occasionally, the Swedish coast to the north. The museum tells the story of the fortress and includes a few artefacts dredged from the harbour. Modest entrance fee, opening hours follow the ferry schedule.