Castel Sant'Angelo
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Castel Sant'Angelo is - and this is where I start shamelessly copying my tourist guide book because I am cut off from Wikipedia - a mausoleum to Emperor Hadrian that was later turned into a fortress. It had prisons, torture chambers, lodging and stores for the soldiers and papal apartments; going from the bottom to the top.
Like so many other places, there is a ban on photography in all the really interesting rooms - in particular the court rooms, which are covered in frescoes from top to bottom.