This is an ancient amphitheater that has been restored and now showcases plays and concerts you can purchase tickets to. For the best picture of this theater you need to hike to the top of the Acropolis and look down over the side - amazing. The Theater of Dionysis is on the southern slope and is not the same - you can actually walk into that and sit in the stands. TIP – if you purchase the Athens historical site combo ticket for the Acropolis that cost is 12€ too, but you also receive entrance to the Roman Agora, Ancient Agora (and museum), Theater of Dionysus (south slope), Kerameikos (ancient cemetery with a museum) and Hadrian’s Library. Each of these sites costs you 2€ more without it so don’t just pay to the see the Acropolis when you can get all these sites for the same cost!