It shows a young woman rather wistfully seated in a mountain landscape next to a unicorn - the fabled beast is one of the heraldic emblems of the Farnese family.
Part Gothic cathedral, part Teutonic fortress, part sultan's palace, its roofline is a riot of spires, beehive water towers and gaudily tiled domes, and its flanks are decorated with heraldic symbols and heroic-scale frescoes of the hospital's benefactors.