The Acropolis (which means literally "high city") was originally the walled defensive citadel of Athens. Later when the city was secure enough not to worry about being attacked, they converted the top of the hill into a precinct for temples. Today there are the ruins of three temples there: the Parthenon, the Erectheion and the Temple of Athena Nike. There are also various other ruins dotted around the approaches to the hill.
This is the must-see sight of Athens, and to top it all, there's a great view of the rest of the city too.
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