The vast Franciscan friary attached to the Basilica di San Francesco — a complex of buildings on multiple terraces propped up by the famous arcade of arches that you see on the approach from the plain. Begun in the 1230s alongside the basilica, it has been continuously inhabited by Franciscan friars ever since.
Most of the friary is closed to visitors, but the upper cloister and the friars' library (with manuscripts from St. Francis's time) can be visited on request. The big interest is the museum and treasury, which holds the Pope Nicholas IV chalice, illuminated manuscripts, and the only contemporary portrait of Francis ever painted from life — by Cimabue, in the lower basilica.