The Orange Garden is the open courtyard of a medieval fortress. The Savelli family held this corner of the Aventine hill in the thirteenth century, and their crenellated brick wall is what you lean on at the western end. In 1932 the city planted the enclosure with bitter orange trees in straight rows and opened it to the public, so the whole garden is one rectangle of dark leaves with a stone parapet across the far end. It is small.
From the parapet the ground drops away to the Tiber and the city spreads out flat to the west. The dome of St Peter's stands up clearly to the right, and the sun goes down to the left of it, over the river and the wooded ridge of the Janiculum. The two are in the same view but never line up, whatever the photographs suggest. On the next square, a gate belonging to the Knights of Malta has a keyhole that frames the dome down a clipped avenue of hedges. That gate is the only building Giovanni Battista Piranesi ever completed.