Hierapolis

sacred city, a city of Phrygia, where was a Christian church under the care of Epaphras (Col 4:12; 4:13). This church was founded at the same time as that of Colosse. It now bears the name of Pambuk-Kalek, i.e., "Cotton Castle", from the white appearance of the cliffs at the base of which the ruins are found.

EBD - Easton's Bible Dictionary