Palamidi Fort, high above the town.
I like towns with upper and lower sides; a bit of geography, topography, up and down bits to keep things interesting. (It was the only decent concept in Dave Sim's warrior aardvark comic: an upper and lower city connected by a staircase so long that if you fell down it you had a major expedition on your hands.)
I must have sweated off the entire 1.5l bottle of water climbing 400-odd steps, but the view is worth it. I've fallen for this little town in a crushing, futile, she's-a-decade-younger-and-dates-guys-with-much-more-money kind of way. There's a three-storey wreck on the hillside near my inn, and I'm seriously considering making 'em an offer.
I think Thermopylae and Delphi are off my itinerary now; there's so much in the Peloponnese and heading for Northern Greece (which means a trip back to Athens) would take three days out of my precious 'miniature gap year'. The events that defined western civilisation happened here, in the Peloponnese: there's enough here to fill the next twelve days.