Castel Sant'Angelo (or "Hadrian's Mausoleum") is a fortress connected to the State of the Vatican through the corridor called "del passetto". Built by Emperor Hadrian in 125 as funeral mausoleum, stands in front of the Campus Martius, which was joined by a bridge (Elio Bridge). The Mausoleum was a cubic base, covered with marble and decorated with a bucrania frieze bearing the names of the emperors deposed. Entry was through an arc entitled to Adriano and a dromos. Around the Mausoleum ran a wall with bronze gate. The present name dates back to 590, when Rome was afflicted by the plague: it's said that the Archangel Michael appeared to Pope Gregory I while leading a procession of prayer right near the Mausoleum. When the Western Emperor Honorius did include the Mausoleum in the Aurelian Walls, the building became the bulwark over the Tiber in defense of Rome, shown for the first time as Castellum. In the eleventh century, it was used as a State Prison, to be fought over the centuries by various noble Roman families. In 1367 the building was given to Pope Urban V to allow his return from the exile of Avignon: from that moment on, the history of the Castle binds those of popes and its architecture undergoes considerable change. Niccolò Lamberti has designed a unique ramp with drawbridge and rebuilt the chapel dedicated to St. Michael the Archangel; in '400 were added the papal residences and three bastions. Antonio da Sangallo the Elder makes it a modern fortress, with four pentagonal bastions dedicated to the Evangelists and a further cylindrical tower; the new papal apartment was frescoed by Pinturicchio and joined with gardens and fountains. In '600, Clement IX had placed ten marble angels on the Elio Bridge, since then called St. Angelo Bridge; in the nineteenth century, the Castle was used as a prison, only after the Unification of Italy it became a museum, "National Museum of Castel Sant'Angelo".