Built in the spot that they believe St. Peter to have been crucified upon, St. Peter's Basilica is the heart of Vatican City. Originally begun in 323 C.E. by Constantine, it supposedly houses an altar above the point of crucifixion, the throne of Peter, and his tomb. It also houses several chapels, which are considered some of the Western world's greatest works of art. Pillaged by Saracen invaders in 847 C.E., Pope Leo IV decided to surround it with a wall and towers, some parts of which still stand. When the papacy fled to Avignon, the Basilica fell into disrepair, and it was decided to build a new one from scratch. Built in a cross shape, it is constructed primarily of vari-colored marble, and decorated with extensive gilding, painting, mosaic, and statuary. It also houses some important relics of the Roman Catholic faith, including what they believe to be a chunk of the cross that Jesus was said to have been crucified upon. The remodeling took  176 years to complete, from 1450 to 1626 C.E. St. Peter's Basilica can be considered the center of the Roman Catholic Church, but even ardent non-theists can appreciate the beauty of the building and the artwork within.