Castel Sant' Angelo (Castle of the Holy Angel) was built in 135 A.D. as a tomb for the Emperor Hadrian. By the 6th Century it had been modified to serve as a fortress. Linked to the Vatican by an arched passageway, for centuries it served as a place of refuge for the popes during times of attack on Rome. The fortress received its present name when Pope Leo the Great in the year 590 had a vision of an angel above the fortress replacing his sword in its sheath. A massive bronze statue at the top of the building depicts this vision. Open to tourists, the Castel is now a military museum with cannons, cannon balls, and a collection of antique weapons and armor.