Diaoyucheng, China

Living in this 47,205-person fortress would be a constant exercise in siege mentality. The Diaoyucheng, as it's known, has a long and storied history of repelling Mongol invaders, but that defensive posture has never really gone away.

Perched on a hill surrounded by rivers, the ancient fortress is a marvel of engineering, with its naval wharf, watch towers, and built-in cannons. But day-to-day life here is a persistent headache, with over 200 military confrontations recorded during the 13th century alone. Outbreaks of dysentery and cholera were common, and the local climate can be unforgiving.

Only the most stalwart and battle-hardened individuals need apply to live in this historic but isolated enclave.