Derinkuyu Underground City in Nevşehir Province, Turkey
Like something out of an especially surreal horror movie, the Derinkuyu Underground City was discovered in modern times when a man discovered a hidden room behind a wall in his home which, after further digging was revealed to lead to an ancient underground complex, 18 stories deep.
Likely first established around the 7th or 8th century BCE, the massive subterranean complex was built by the ancient population of the time to provide protection against invading forces. And what a defense it became. Continually growing since its inception, and significantly expanded during the Byzantine period centuries later, the hidden "city" is thought to have been able to protect 20,000 inhabitants for long periods of time thanks to a number of surprisingly advanced, if crudely implemented innovations. The city descends 18 stories underground, deeper than any of the other underground complexes found in the area, and is fitted with thousands of ventilation shafts and water ways that provided fresh air and water to each level of the site. There were rooms for stables, churches, lodging, storage, and of course a winery, lest the citizens become bored during a siege. The city was protected by doors made of massive stone wheels that could be rolled in from of an entrance, essentially making it another wall.
Today, over 600 entrances to the Derinkuyu Underground City have been found in courtyards and private residences around the city giving the impression that there may have been some holes in this impressive defense, but hundreds of entries or not, it is doubtful that many enemies could make it past two-foot stone doors.