Greece's Monastic Republic of Mount Athos is an autonomous state within a state, like Vatican City within Italy. Today, it comprises 20 monasteries housing 2,000 Eastern Orthodox monks. The Virgin Mary personally set foot there, and asked for Mount Athos to preserved as a garden for people seeking salvation. Women have been forbidden from entering for at least a millennia.
Exceptions to this rule are few and interesting: when a Serbian emperor brought his wife to Mount Athos to protect her from the plague in the 14th century, for example, it's said she was carried around constantly by hand carriage, so her feet never touched the ground.
Though Mount Athos has been inhabited since perhaps 800BC, it was primarily self-isolating hermits until Athanasius the Athonite founded its first and largest monastery in 963. His fortress-like Monastery of Megisti Lavra was intended to protect Mount Athos's scattered Christians from potentially invading Arab Muslims. He was killed, accidentally, by a collapsing church arch.