Built between 135 and 139, as the tomb of the Roman Emperor Hadrian, the Castel Sant'Angelo was designed to be slightly larger than the Mausoleum of Augustus, because, as this database has shown time and again, size does matter.
The building was converted into a military fortress in 401 and was promptly sacked by Visigoth looters in 410, who scattered Hadrian's ashes everywhere. What was left was recycled, i.e. stolen by the Vatican.
In the 14th Century, the tomb as converted into a castle for the popes and connected to St. Peter's via a covered passage called the Passetto di Borgo. Reflecting the delightfully sadistic side of Renaissance papal rule, the castle contained both sumptuous apartments and a prison. Giordano Bruno, the famous scientist and Hermeticist, was held there for six years. Executions were carried out in the small interior courtyard for the Pope's enjoyment.