Facing the Duomo across the cathedral square, Santa Maria della Scala is now a single ticketed museum covering several floors and centuries of accumulated buildings. The highlights are the Sala del Pellegrinaio (frescoes of medieval hospital care by Domenico di Bartolo), the original marble reliefs from the Fonte Gaia by Jacopo della Quercia, and the underground Etruscan collection in the deep cellars.
Allow at least two hours; it is genuinely vast, and the lower levels are often empty.