They write different things about this building on the Web. Someone started the speculation that this is an ancient infirmary/hospital. 100٪ there are no confirmations. The building shows that it was rebuilt (by the way, it is everywhere in Side). There are antique masonry and structures, something was completed in the Middle Ages. How the infirmary could be used. Moreover, the capital of one of the Crusader states was nearby in Antalya. By the way, there are many crosses of soldiers of Christ in the museum nearby. Apparently, they were buried next to the building.
The feeling is unforgettable. To touch history with your hands.