Colosseum

The colosseum, a roman structure used for "celebrating, sports events, and bloodshed" is a massive structure built in 70AD and over half still stands today. The outer south wall, having been destroyed in an earthquake, is gone completely, but it almost makes the place so much cooler. Instead of it being an enclosed oval, it's half open so you can see south from the north side. It's an iconic place of history, and a must see if you're ever in Rome!