Panagia Koumpelidiki is one of the most distinctive churches in Kastoria and arguably the one most worth seeking out if you are short on time. Probably 10th or 11th century in origin, it stands in the old part of the peninsula town with a tall cylindrical dome drum that makes it immediately recognisable — its profile is unlike any other church in the region.
The interior is small but covered in Byzantine frescoes from different periods, with the earliest layers visible where later paintings have flaked away. The church demonstrates the long continuity of religious life in Kastoria: it has been repainted, restored, and used continuously for roughly a thousand years.
Like many Kastoria churches, it is locked outside of service times, and you may need to ask locally for the key. The church is walkable from the Byzantine Museum and forms part of any natural circuit of the old town.