The Passetto di Borgo is an elevated 1/2-mile-long passageway that links the Vatican with Castel Sant'Angelo built in 1277 to serve as an escape route for a Pope in danger. In the 1st photo below, taken from the top of St. Peter's Basilica, it is the curved brown strip running out to Castel Sant'Angelo, behind the buildings on the left side of the road in the upper third of the photo. In the 2nd image below, taken from Castel Sant'Angelo, it's the walkway on top of the brown wall near the right edge. The 3rd and 4th images below show the Passetto at ground level. Pope Alexander VI used it in 1494 when Charles VIII invaded the city. Clement VII escaped to safety through this walkway during the Sack of Rome in 1527 when troops of the Holy Roman Emperor, Charles V, massacred almost the entire Swiss Guard on the steps of St. Peter's Basilica. Out of it's 500 members, only 42 escaped the onslaught and managed to smuggle Pope Clement VII through the Passetto to Castel Sant'Angelo. The Passetto played a key role in Dan Brown's novel Angel and Demons.

    

The Passetto di Borgo is the curved stripe running to the Castel Sant'Angelo behind the buildings on the left side of the main road leading out of St. Peter's Square
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The Passetto di Borgo is on top of the wall leading out of Castel Sant'Angelo, at the right edge of this photo
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Passetto di Borgo, crossing a road near St. Peter's Square
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Passetto di Borgo, near Castel Sant'Angelo, at night
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• My Passetto di Borgo photos