La Torre de los Huesos (The Tower of the Bones) – Calle de Bailén, 8, Madrid, Spain

You wouldn’t normally expect to find one of the most ancient towers in Madrid in the heart of an ordinary parking garage. According to Atlas Obscura, should you choose this spot to park before a visit to the Royal Palace, you will see the remains of an 11th-century tower built by the Moors. It got its name as it was close to the ancient Islamic cemetery of Huesa del Raf. There’s no entrance fee, just the normal price for parking.