One of the best-preserved fortresses along the Croatian coast is a treat for urban explorers. Connected to land by only a narrow walkway that just barely rises above the water, its arrow-like shape suggests that this fortress was not one to be messed with. The structure’s seemingly impenetrable exterior hides large, looming brick hallways that beg to be explored.

The Venetians built St. Nicholas Fortress in the 16th century to block maritime Turkish attacks and keep any invaders from coming ashore. Constructed from a sturdy blend of brick and stone, its triangular shape and 32 resident cannons made it a truly formidable form of defense.

In fact, it looked so foreboding and impenetrable that no one even tried to test it. Its appearance and reputation alone were enough to deter any would-be attackers from tempting their fate. Yet despite its lack of action, the military continued to maintain the fortress for centuries until they eventually abandoned it in 1979.

Now, only urban explorers infiltrate the stalwart structure. Most make their way inside via the land, though those with access to a boat can try their hand at squeezing in through the sea-facing entrance. The fortress’s surprisingly airy interior is remarkably well-preserved, making it easy to imagine you, too, are a member of one of the various armies that occupied its walls during its many centuries of use.