Caste Sant´Angelo 

built by Emperator Hadrian as a mausoleum in 130 AD for himself and his successors. The caste is divided into 5 floors.
1st - famous winding ramps
2nd - floor of the prisons
3rd - military floor
4th - papal floor
5th - the top floor with a terrace from which you can see a panorama of the city.

Protecting the Pope - The Vatican Corridor leads from the Vatican Palace to Castel Sant' Angelo. It was built in 1277 to provide an escape route when the pope was in danger.