The gradual fortification of the structure was accelerated when the papacy purchased Hadrian's tomb in 1277. The walls were strengthened and heightened, and a second defensive system was put in place around the outer perimeter in the shape of a square. The popes were concerned, at the time, about their safety in Rome, and wanted some kind of fortress in case of invasion. This is the view from the top of the walls, with the basilica rising against the sky on the left. A long wall is visible on the right - this contains a passageway, called the "Passetto di Borgo," which connects the Vatican's main complex to Castel Sant'Angelo. It was used infrequently for actual escape, though I am told that it is featured in several works of fiction. I have never read Dan Brown, but apparently everyone else has and knows about this walkway.
The upper walls enclose a pretty little collection of courtyards and buildings - there is now a cafe and an art museum housed in the castle. It's a little warren-like, and it's easy to miss the directional signs, but the sun was pleasant and the views out over Rome are spectacular.
The popes made many renovations that were non-military in nature, and the uppermost spaces are beautifully decorated with frescoes and plaster moldings. Inside, where the art museum now is, the rooms are nearly as fantastic as those in the Vatican museums. Photography is prohibited, but I can assure you that it's striking. This is certainly unlike any castle I've ever seen. There is, for example, a church on site that was designed by Michelangelo.
The Sant'Angelo bridge was also erected by Hadrian, in part to access his new tomb. It has survived remarkably well, and is now decorated lavishly with renaissance-era statues. Here it is, stretching across the muddy, springtime Tiber.





