Krakow's medieval fortifications
The Krakow Barbican is what remains of Krakow's medieval fortifications and is one of the few buildings of its kind which is still standing in Europe. Built in 1499 in response to the Ottoman invasion, the Barbican has a circular diameter of 25 meters surrounded by a stone and brick wall several meters thick and a moat. At the Barbican itself, there isn't much to do besides climbing the stairs, but you can make a combined visit too the Barbican and the city walls which makes for a pretty fun time. It's like traveling back in time. The views from the watchtowers are excellent.