A tiny, plain Romanesque church on the steeply sloping Via Santo Stefano, a few minutes' walk west of the cathedral. The single-naved interior is essentially unaltered from the 12th century, the masonry exposed and the windows narrow.
The location, on a quiet residential street with vines climbing the houses opposite, gives a strong sense of medieval Assisi without the crowds. Easy to miss; ten minutes to visit. Worth the detour.