Château de Najac ~ a royal fortress, built in 1253, which has seen "events of...the first English occupation, the Albigensian Crusade, the Hundred Years' War, the imprisonment of the Knights Templar, the peasants' revolts, and the French Revolution. ~ Southern France
Duomo di Sienna, Italy - OK, so I'm a total sucker for a good ceiling. And this is a very, very good ceiling. Seriously though - give me a gorgeous Gothic ribbed vault over the shiniest skyscraper in London, any day. It ages better, and it's an equal if not greater marvel of engineering.