Basilica San Clemente (Rome) - An incredible 12th century church, built on top of a 4th century church, built on top of a 1st century church.  Near the Colosseum.

1st picture is from the Basilica’s website, 2nd and 3rd pictures are from Flickr user allie_caulfield and are Creative Commons licensed

http://www.basilicasanclemente.com/