Called the Red Hall (Kizil avlu) this is a Roman sanctuary, maybe from the second century AD. It is thought it might be a Serapeion or Temple for Serapis, which is suggested by the statues that were found an the proximity of water: the river Selinus flows under it, through two tunnels. In Byzantine times it was transformed into a basilica with three halls. Part of it is still used as a mosque.
A commentator calls this the Red Church, and she is not alone, however, the Lonely Planet calls is it Red Basilica.