It is one of the most important monuments of the 14th century. Agios Athanasios was built in a period of time that was quite critical for the city of Kastoria. The situation in the wider region was no longer controlled by the Byzantines and the change in power of local rulers was easy. Thus, around 1387, the noble family of Mouzaki, of Albanian origin, appeared in Kastoria and took over the rule of the city from the Serbs. The architecture of the building is quite simple and belongs to the type of the single-aisle basilica that ends in a semicircular niche to the east. To the west the building is completed with a later narthex.
The roof is wooden and is not the original one. It was replaced in the 1950s, retaining several elements of the original. The masonry is composed of irregular stones bonded with mud. Externally, the facades of the church have no particular aesthetic value, except for the blind arches, two on each of the north and south sides and two on the west side to the right and left of the entrance to the church.