The Passetto, or "er Corridore" (the corridor), is the Vatican walls that connects the Vatican with Castel Sant'Angelo.
It was built by 'anti-pope John XXIII (Baldassare Cossa) in the fifteenth century. The walls were in poor condition and in the process of restoration was thought to build a passage leading to the castle directly from the Vatican.
The purpose was to allow the head of the church to take refuge in case of necessity into the castle and at the same time have a stronghold that would allow better control of the district.
On the front we see the carved symbol of the papacy.