The Pantheon sits atop the Acropolis and is a fantastic site to visit. It is a hard climb on a hot day and on the day of our visit it was 30 degrees. However, it is so worth the climb, The history is incredible and the 360 degree views of the city of Athens are breathtaking. We visited on a Tuesday morning and as we went to pay our 20 euro entrance fee, were told that it was free and we were handed two free tickets. Don't know if this a regular Tuesday thing, but may be worth asking. We arrived there at around 10.00 and it was VERY busy.