Castel Sant'Angelo

Hadrian's tomb went up on the right bank of the Tiber in the 130s and was completed under Antoninus Pius in 139: a square base faced in Luna marble, a travertine-clad drum above it, and on top a planted mound crowned by a bronze quadriga driven by Hadrian in the guise of the sun. At some 50 metres to the top of the mound it overtopped everything else in the city.

His ashes were placed here a year after he died at Baiae in 138, with those of his wife Sabina and his adopted son Lucius Aelius. Emperors kept arriving after him — Marcus Aurelius, Commodus, and Septimius Severus with his wife Julia Domna — down to the last recorded deposition, Caracalla, in 217. Hadrian also built the Pons Aelius to run straight onto the mausoleum.

The tomb stopped being a tomb at the turn of the fifth century, when it was absorbed into the city's defences beyond the Tiber: converted to a fortress around 401 and taken into Honorius's Aurelian Walls by 403. From that point it was called a castellum. In 410 Visigothic looters scattered the urns and ashes during Alaric's sack of Rome, and the decorative bronze and stone statuary was thrown down on attackers from the walls — Procopius describes this against the Goths besieging Rome in 537, while another tradition places the same defence during Alaric's sack and Genseric's Vandals in 455. The Barberini Faun was later recovered from the fortress ditch. By the early sixth century Theodoric was using it as a state prison.

The name arrived in 590. With Rome under plague, Pope Gregory I led a penitential procession, and crossing the Aelian bridge he saw the Archangel Michael on the summit sheathing his sword — read as a sign the epidemic was ending.

In the first half of the tenth century the castle was the stronghold of the senator Theophylact, then of his daughter Marozia, who used it as a prison — a function the building kept until 1901. In 932 Marozia held her third wedding, to Hugh of Provence, in the imperial burial chamber itself; during the banquet her son by her first marriage, Alberic II, appeared in the castle, forced Hugh to flee and seized power. Marozia ended her days obscurely in one of its cells.

The Crescenzi took it in the second half of the tenth century and strengthened it so thoroughly that their name attached to the building: Castrum Crescentii, which it carried long after passing to the Pierleoni and then the Orsini. Pope Nicholas III, of that same family, judged the Lateran Palace unsafe and moved part of the apostolic seat here, building the Passetto di Borgo, the covered fortified corridor to St Peter's. The Orsini ceded the castle to the Church around 1365, and in 1367 its keys were handed to Urban V to press him to return from Avignon. In 1379 a Roman crowd, enraged at the French garrison Urban had left in it, came close to levelling the place; Boniface IX began the rebuilding in 1395 under the military architect Niccolò Lamberti.

Nicholas V (1447–1455) installed the first papal residence inside the building and raised three bastions at the corners of the outer quadrilateral. Alexander VI went further, setting Antonio da Sangallo the Elder to work: four pentagonal bastions named for the Evangelists swallowed those earlier structures, a cylindrical tower went up at the bridgehead, and a ditch round the walls was flooded with Tiber water.

Those works paid off thirty-two years later. Charles V's Landsknechte opened the Sack of Rome on 6 May 1527, and Clement VII, sheltering in the castle, held out through a seven-month siege. In 1542 Paul III had the place remodelled by Raffaello da Montelupo and Antonio da Sangallo the Younger, the apartment he built intended to give the pope somewhere fit to stay in any future siege. The pentagonal bastioned enceinte was begun under Paul IV (1555–1559) and completed by Francesco Laparelli under his successors; in 1630 Urban VIII destroyed all the earlier fortifications, including the Borgia tower between bridge and castle.

The worst cell, the Sammalò behind the bastion of San Marco, had been one of four ancient shafts giving air to the imperial burial chamber before the Middle Ages turned it into a dungeon: the condemned man was lowered in from above and could neither stand up nor lie down. Executions were carried out in the small inner courtyard. Beatrice Cenci was held here, Giordano Bruno for six years, and Cardinal Giovanni Battista Orsini on the charge of trying to poison Alexander VI. Benvenuto Cellini, confined between 1538 and 1539, lowered himself from the top of the enceinte on a rope of bedsheets during a feast and broke a leg in the fall; recaptured, he was shut in the escape-proof segrete, in the cell where the preacher of Foiano had been starved to death, and pardoned a year later.

The nineteenth century narrowed the castle to a political prison under the name Forte Sant'Angelo, and it was decommissioned in 1901. After unification it served as a barracks before the Royal Army's engineers, led by Colonel Luigi Durand de la Penne and taking up an idea of Captain Mariano Borgatti's, set about returning the monument to its ancient appearance, stripping two thousand years of accumulated fabric to do it. A museum of military engineering opened on 13 February 1906 with Borgatti as its first director; the building is now the Museo Nazionale di Castel Sant'Angelo.