Built as a hospice for pilgrims travelling the Via Francigena, this huge complex opposite the duomo dates from the 13th century. Its highlight is the upstairs Pellegrinaio (Pilgrim's Hall), featuring vivid 15th-century frescoes by Lorenzo di Pietro (aka Vecchietta), Priamo della Quercia and Domenico di Bartolo. All laud the good works of the hospital and its patrons; the most evocative is di Bartolo's Il governo degli infermi (Caring for the Sick; 1440–41), which depicts many activities that occurred here.
There's so much to see in the complex that devoting half a day is barely adequate. Don't miss the hugely atmospheric National Archaeological Museum set in the basement tunnels; the medieval fienile (hayloft) on level three, which showcases Jacopo della Quercia's original 1419 sculptures from Siena's central Fonte Gaia; and the Sagrestia Vecchia (Old Chapel) of the Chiesa SS Annunziata to the right near the main entrance, which houses di Bartolo's Madonna della misericordia (1444–45) and a fresco cycle by di Pietro illustrating the Articles of the Creed.
There's an excellent gift shop on-site, as well as a pleasant cafe that can be accessed from Piazza Duomo. Entry is included in the Acropoli and joint Museo Civico/Santa Maria della Scala passes.