A vast circular bastion at the south-east corner of the town wall, built in 1517 to withstand the new gunpowder artillery of the day. The Zwinger's walls are six and a half metres thick at the base, and the tower itself is unusually low and squat — the design philosophy was to absorb cannon shot, not deflect it. From the outside it looks like a stone barrel half-buried in the earth.
Inside, four floors of cramped rooms hold a private museum on late medieval and early modern arms, armour, and torture instruments. The collection is not for everyone — some of the implements are reconstructions of nastier devices — but the building itself is worth the entry fee, with original spiral staircases and arrow slits intact. Climb to the roof for a quick view over the Gose valley and the town walls.