At the end of the fourteenth century Castel Sant'Angelo became an impregnable fortress, when Pope Boniface IX had a deep moat dug around the Mausoleum. As Biondo Flavio recounts in the mid-fifteenth century, the pope transformed it into «the most fortified of fortresses».

Over the centuries the fortifications of the Castle were strengthened and enlarged several times, adding various pentagonal-shaped walls and bastions all around, and surrounding it with a large moat full of water.

The discovery of gunpowder completely changed the methods of warfare, especially as far as sieges were concerned: the walls had to be redesigned and strengthened to resist cannon fire. The besiegers armed with battering rams, ladders and ropes simply disappeared.

In 1527 the Sack of the Landsknechts devastated the city and pope Clement VII locked himself in the Castle for seven months.
Mindful of that tragic experience, pope Urban VIII Barberini decided to further fortify the Castle by building a new wall and moving the entrance on the side, and wanted to equip it with new and more modern cannons.

The operation was very expensive, and to save money the pope did what others had done before him: ordered to plundered the metal from the ancient Roman buildings.
But to make cannons it was not enough to make holes between the marble blocks or in the column drums, collecting just small metal clamps or lead fistulas; it wasn't enough to steal the bronze tiles that covered the ancient buildings, as had been done for centuries. An enormous amount of metal was needed.

Therefore the choice fell on one of the grandest monuments inherited from Roman antiquity, the Pantheon, whose gilded bronze tiles had been stolen centuries earlier.
Until then, the bronze beams that supported the roof of the portico were still intact; Urban VIII ordered to remove and melt them, getting over two hundred tons of metal, of which only a few rivets survive, weighting 15 kgs. each.

A big scandal broke out, and the Pope tried to calm it by claiming to have used that bronze for Gian Lorenzo Bernini's canopy in St. Peter's Basilica.
But archival documents discovered by scholar Louise Rice prove that the bronze was only used to make the cannons, more than eighty pieces.

You can read more about this fictional story in Marina De Franceschini's new book «Castel Sant'angelo. Mausoleo di Adriano. Architettura e Luce» and also in her previous book «Pantheon. Architecture & Light».