Look closely at the stonework around the Sankt Goransporten gate on the eastern side of the wall and you will see a roughly cannonball-sized lump of darker stone embedded in the masonry. It is a catapult projectile from one of Gotland's medieval civil wars, hurled at the city sometime in the 13th century and never extracted.
It is a small thing, easily missed, but it captures something essential about Visby — that the entire city is a working artefact, with the violent past not behind glass but underfoot and overhead. A small information panel marks the spot.