It was rebuilt and adapted to become the fortress and dungeon of the Renaissance Popes. The interior is very lavishly decorated: note the papal coat of arms in the ceiling!
From the battlements there is a magnificent view of St. Peter's. The Vatican palace is to the right of the cathedral
The defence of the Castel Sant' Angelo was organised and led by the great goldsmith Benvenuto Cellini, or so he tells us in his memoirs. Of course, St. Peter's at the time would have looked nothing like it does today: in 1527 it was no more than a building site.