In this neighborhood of huge, austere palaces, Palazzo Spada strikes an almost frivolous note, with its upper stories covered with stuccos and statues and its pretty ornament-encrusted courtyard. While the palazzo houses an impressive collection of Old Master paintings, it is most famous for its trompe l’oeil garden gallery, a delightful example of the sort of architectural games rich Romans of the 17th century found irresistible. Even if you don’t go into the gallery, step into the courtyard and look through the glass window of the library to the colonnaded corridor in the adjacent courtyard. See—or seem to see—Borromini’s 8-meter-long gallery quadrupled in depth, a sort of optical telescope taking the Renaissance’s art of perspective to another level, as it stretches out for a great distance with a large statue at the end. In fact the distance is an illusion: the corridor grows progressively narrower and the columns progressively smaller as they near the statue, which is just 2 feet tall.