The Chapel of Sao Sebastiao is small, fortified, and very Alentejo: white walls, defensive-looking merlons, and a plain exterior that gives little away. Inside, the vaulted Gothic ceiling has frescoes of musical angels.
It is the kind of minor monument that makes Alvito worthwhile. You would not cross Portugal for it, but it rewards a detour if you are already between Beja and Evora.