Castel San Angelo, Rome

Here is the building that has been successively a mausoleum, a barracks, a prison and a museum. Originally it was the Emperor Hadrian's Mausoleum, complete with trees surmounting the mound.

It then became a Papal strong-hold offering refuge to Pope's who could flee by an elevated corridor to the the Castel. It also housed a Papal prison in the 19th Century.