Fronting an ornate cypress-shaded piazza, the Roman headquarters of the Sovereign Order of Malta, aka the Cavalieri di Malta (Knights of Malta), boasts one of Rome's most celebrated views. It's not immediately apparent, but look through the keyhole in the villa's green door and you’ll see the dome of St Peter’s Basilica perfectly aligned at the end of a hedge-lined avenue.

　　Unfortunately that's probably as close as you'll get to entering the building, which can only be accessed with special permission or during rare special openings.

　　The piazza, which sits on the summit of the Aventino hill, owes its ornamental look to a late 18th-century overhaul by the artist and architect Giovanni Battista Piranesi.