Hierapolis was an ancient city located in southwestern Turkey near the city of Denizli. It was founded in the second century BC by the Attalid kings of Pergamon as a thermal spa town, and its name translates to "sacred city". The city was built around hot springs that were believed to have healing properties.

One of the most significant features of Hierapolis was its large and well-preserved theater. The theater was built in the second century AD and could seat up to 15,000 people. It was designed in a Hellenistic style and was used for performances, gladiator contests, and other public events.

Another famous feature of Hierapolis is the Pamukkale hot springs, also known as the "Cotton Castle". The hot springs are located on a hill above the city and have been used for thousands of years for their healing properties. The mineral-rich water flows down the hillside and forms white terraces that look like frozen waterfalls.

Hierapolis was also an important center for early Christianity. The Apostle Philip is said to have preached in the city and was later martyred there. The city is home to several ancient Christian ruins, including the Martyrium of Philip, a large octagonal structure built in the fifth century to commemorate his death.

Today, Hierapolis is a popular tourist destination and is recognized as a UNESCO World Heritage Site. Visitors can explore the ancient ruins, including the theater and the Martyrium of Philip, and relax in the Pamukkale hot springs.