T his castle started as a place of death, and eventually found a role preserving life. Castel Sant'Angelo was built as a tomb for Emperor Hadrian in 135. It continued to be used as a mausoleum until sometime after 500 when it started being used as a fortress. The castle occupies a strategic point on the Tiber River and was considered the military key to Rome. More importantly, it served as the first line of defense for attacks against the Vatican. It is connected to the Vatican by an tunnel called the Passetto. This was used during the Sack of Rome in 1527 when the Pope fled the attackers and hid inside the castle. By then it had been transformed from a fortress into a papal retreat where pontiffs sought refuge for 1,000 years. It was a pope that gave the fortress it's current name. In 590 Gregory the Great was leading a procession across the bridge praying for an end to the plague when he had a vision. It was an angel putting his sword away while standing on stone ramparts. Touched by this vision, Gregory ordered a stone statue to be placed on the castle. That statue was replaced by a bronze one in 1753, and it still stands there today, frozen in time. Inside, it has been transformed into a museum. There are dungeons, cannons, stone cannonballs, armor, and antique weapons. The Papal Apartments resemble more art museum than history museum, and are richly decorated. You can even see the 16th century war chests the used to store the Vatican's treasures in times of emergency. This is also the location where, in Puccini's opera, Tosca throws herself from the ramparts crying, "Scarpia, avanti a Dio!" ("Scarpia, we meet before God!")