The mausoleum was gradually transformed into a castle. Theodoric, the king of Italy (493-526), used it as a prison and for a time it became known as carceres Theodorici.

According to the legend, St.Gregory the Great, while crossing the Pons Aelius (the bridge Sant'Angelo) at the head of a procession to pray for the cessation of the plague of 590, saw on the top of the fortress an Archangel Michael sheathing his sword. The vision accurately announced the end of the plague and from then onwards the castle has born its present name.

The funeral function of the construction was lost completely when Leo IV erected his town-wall around Vatican and Borghi, leading it from St Peter's to Castel, so it became a fortified defense of the pope's city.

By the late 12C the castle was established as papal property. In 1378 it was severely damaged by the citizens of Rome, resentful of foreign domination. In the reign of Boniface IX rebuilding begun. Alexander VI had A.da Sangallo the Elder complete the four bastions of the square inner ward which had been begun by Nicolas V.

Julius II built the south loggia, facing the river. When Clement VII and some 1000 followers (including 13 cardinals and 18 bishops) took refuge here in 1527 from the troops of Charles V, Benvenuto Cellini took part in its defense. Paul III built the north loggia and decorated the interior with frescoes. The outer ward, with its defensive ditch, was added by Pius IV. Urban VIII provided the castle with cannon made of bronze taken from the ceiling of the Pantheon portico, and he employed G.L.Bernini to remodel the outer defenses.

From 1849 to 1870 the castle was occupied by French troops. Under the Italian Government it was used as barracks and as a prison until 1901, when the work of restoration was begun. In 1933-34 the castle was adapted for use as a museum (58rooms) and the surrounding area was cleared. According to the tradition the fireworks were invented by Michelangelo and brought to perfection by G.L.Bernini. Till the end of 19th century the fireworks were activated on the holidays and special occasions. Castel S.Angelo also had the enormous machines producing fantastic designs in the air with the colored fires.

 

From the end of 11th century the Angel of castle was substituted 6 times.

The 1st statue was made of wood and was destroyed by time.

The 2nd was marble one and was destroyed during the attack on the castle in 1379.

The 3rd one was also from marble but with the bronze wings and in 1497 was blown in the air by the lightning.

The 4th was made of gilded bronze, which ended by being used as a material for cannon in 1527.

The 5th (see on the left) was created by R.da Montelupo in 1544 from marble with bronze wings, it was on the tower top till 1752 as was destroyed by the weather, now it is in the Courtyard of the Castle.

The 6th angel (see on the right), which is 4m high and made of bronze, belongs to the work of Flemish sculptore Pieter Van Verschaffelt; in 1798 the French depicted this angel with the colors of the Revolution (white-red-blue), put on its head the Phrygian cap and gave it the new name "Genius of France-liberator of Rome". The statue was restored in 1983-86.

Being an official prison for many centuries Castel S.Angelo contains numerous prison cells, to begin with those built by Pius IV with the sizes that wouldn't permit a prisoner to stand on the legs and lie, the only position possible was sitting.

 

In the so called Cortile Dal Pozzo, built by Alexander VI, were the cells reserved for important persons. For example, here in 1538 - beginning of 1539 for the first time was held Benvenuto Cellini, where he could continue his activity as a sculptor and from where he escaped using the rope made of the pieces of bed sheet. But he was caught in a short time and re-imprisoned again till the end of 1539, but this time - in underground cell.

In the loggia over the pope's apartment of Paul II is a room where in 1789 was kept the famous adventurer Giuseppe Balsamo named count of Cagliostro. 

The processes over the prisoners were held in the Hall of Justice, it was built in Roman times above the sepulchral cella, and has a fresco of Justice attributed to Domenico Zaga.              

The executions took place out of the castle on the other side of the Tiber, but in a lot of cases -inside of the castle, right in the cells.

Among numerous historical figures ended their days inside of the castle were 5 popes.

In 897 by the cause of popular rising Stephan VI was sent here and strangled.

In 903 Leo V was imprisoned and killed here by the order of anti-pope Cristopher.

In 928 John X was deposed and strangled with a pillow.

In 973 Benedict VI was deposed and strangled by anti-pope Boniface VII during the popular rising.

In 984 John XIV was imprisoned and poisoned by the same anti-pope Boniface VII.

Alexander VI Borgia was particularly cruel with the personalities standing on his way. So, the cardinal G.Orsini found himself in the prison cell of the Castel Sant'Angelo in February of 1503. His mother and lover (dressed up as a man) found the opportunity to speak with Borgia and offer him a very rare pearl of the greatest value in change of  Orsini's freedom. The pope took it and assured women to make the prisoner free. He held his word and made the cardinal get out of the castle...but when Orsini was already dead.

A couple of months later another cardinal G.Michiel, grandson of Paul II was imprisoned by Borgia and poisoned in the walls of Castle. Specially interesting is a story of a medical doctor and alchemist G.F.Borri (1627-95) imprisoned because of "magic" by the Holy Inquisition. He cured the French ambassador d'Estrees inside of the Castel Sant'Angelo and obtained the fame of a doctor of aristocracy. He was given a little apartment in the prison where he could receive numerous noble clients. Even the pope Innocent XII used his medical help and granted Borri with the freedom. But the doctor was already ill with malaria and died in the castle.

The Hall of the Library and Room of the Secret Archives or the Treasury - here were kept the most important documents about Church life and a treasure collected by Sixtus V for extraordinary needs of Papal State.

The terrace on the top of the castle is the place where the last act of Puccini's opera Tosca was scened.