This is as we approached the Vatican on the first day - behind the trees is the round fort that sits on the Tiber that shows up in tons of photos everywhere.  We took Lorraine Moeller's advice and avoided going inside because of the complete lack of anything describing what was what.  The fortification itself was rather interesting - sort of a super miniaturized Tower London - complete with a dry moat.  A Bizzillion cicadas lived in the trees and were having fun making lots of noise.