Front of Pantheon with detail of the Pantheon Fountain (1575 by Della Ponta). 
The Pantheon was one of my favourite buildings in Rome. It was built by the Romans in the 2nd century AD as a temple, but turned into a catholic church in 609 AD. Bernini even dared to add two clock towers to it, but after a lot of mockery they were removed again in 1883.