The medieval town's star-shaped walls are geometrically dazzling, though not entirely impenetrable.

An expansive complex of subterranean tunnels is all that remains of one of Europe's most formidable medieval fortresses.

One of the only Florida forts to remain in the hands of the Union all through the Civil War still stands as a historic stronghold.

The remnants of the fortress that saved Georgia from the Spanish still stand guard on their swampy island.

Pirates had to be ousted from the land where this now ruined Louisiana fortress was built.

Anyone wanting to know what a dwarven mine sounds like need only visit the tunnels of this New Zealand artillery compound.