There's a 12th-century castle with a "secret" entrance hidden in plain sight - Apparently.
The 12th-century Krak des Chevaliers castle in Syria, a UNESCO World Heritage Site, has a hidden entrance that was cleverly disguised as a window. Archaeologists discovered the entrance in the 1930s, and it's believed to have been used by the castle's defenders to sneak in and out undetected. The castle's unique architecture and hidden entrance made it nearly impregnable, and it's considered one of the best-preserved Crusader castles in the world.