Knights of the Round Fortress

Only a handful of such fortified outposts still exist in Europe, but the Barbican of Krakow is by far the best preserved and most interesting of the bunch. This circular Gothic structure -- built using Arabic defense methods instead of European -- is more than just a pretty fence. It is actually the location for lots of fun activities.

For starters, the 15th-century structure hosts knights' tournaments in which participants sword fight in competition for the main prize, the Barbican Cup. The event also features medieval dramatizations of torture and executions. The Barbican is a division of Krakow's Historical Museum, so its courtyard often features open-air exhibits, and it's a popular place for live concerts in the summer. If nothing else, visitors love to wander around its inner courtyard and turret-lined walls that give it a fantasy book-type of appearance.

The Barbican, which sits just north of the town square, was the gateway to the once-walled city half a millennium ago. Today, it stands as one of the best architectural examples of the circular fortresses that once dotted the European lands.