The Temple of Apollo had an oracle and was tended by priests. The source of inspiration for the Temple was the Plutonium spring located nearby, which gave off toxic vapours. The priests would use this to their advantage by demonstrated the potency of the mythological Hades, by sacrificing small animals into the Plutonium.
Only the foundations of the original Temple of Apollo remain. A new Temple was built in the 3rd century using a Roman style. The marble was recycled form the Temple of Apollo, but again, only the foundations remain.