Mycenae

The citadel on the acropolis.

I really had very little idea of the historical—as opposed to mythological—significance of Mycenae (apart from remembering it as the place where Schliemann found that cool gold mask).

In fact the period from 1600-1100 BC in Greek history is called Mycenaean because the city was the major power in southern Greece. By the end of this period, though, it was part of the general Bronze Age Collapse in the eastern Mediterranean, and though it was partially rebuilt and had a brief renaissance during the Hellenistic era, its glory days were over. It's been a ruin for far longer than it was a thriving city.