North of the Tiber, a few blocks from the Vatican, looms the enormous Castle of the Angel. Originally known as The Mausoleum of Hadrian, the castle began to be constructed the Emporer Hadrian in 135 A.D. Castel Sant'Angelo has been many things in its tenure from Roman to religious building including a fortress, museum, a prison, and a papal residence.
Entering the tower (for a couple dozen euro) one will see the wonders of the building's interior. The castle houses a part of the National Roman Museum's collection and features numerous pieces of art within its walls. The crisscrossing stairs of the interior will eventually lead you out of these museums and to the upper levels with gorgeous views of Rome.
The castle is an iconic part of Rome. It is featured art, literature, and popular culture from Assasin's Creed: Brotherhood to Dante's Inferno. It's well worth visiting, especially as it is so easy to reach from Saint Peter's Basilica which hardly any pilgrims are likely to miss.
Fodor's See It Italy by Fodor's Travel Publications
The Complete Idiot's Guide to the Roman Empire by Eric Nelson