First founded by King Eumenes II of Pergamon soon after 190 BC, Hierapolis was originally a fortified military colony. The original city was destroyed by an earthquake in AD 60, and it was after the rebuilding that its glory days began 

The city enjoyed its greatest prosperity during the 2nd and 3rd century when, with its on-tap natural hot springs, it became an important spa center. The remains of a grand colonnaded street runs parallel to the travertines below for just over one kilometer, extending between the necropolis to the north and a Byzantine church at the southern end

From the church, if you take the eastern path, you come to the Temple of Apollo and its famed Plutonium (a cave beneath the temple that was a source of poisonous gas). Here, the priests would consult the oracle, bringing in birds and small animals killed by the rising gas

Today, nothing much survives of either. East from the remnants of the Agora is the octagonal Martyrium of the Apostle Philip, built on the spot where the saint and his children were supposedly martyred after he remonstrated with the pagan-worshippers of Hierapolis