The cult of Asclepius was already practiced in Pergamon in the 4th century BC. And was initially hereditary in the family of a certain Archias, the founder of the cult. Under Eumenes II it was elevated to a state cult. Access to the Asklepieion was made possible by an 820-meter-long, splendid street, at least in its first part, if you had passed through a gate building that formed the entrance, when via tecta was vaulted.
The Asklepieion in its design known today can be traced back to an expansion in the time of Antoninus Pius. The Roman sanctuary was a courtyard surrounded by buildings and halls measuring 110 × 130 meters, with a large forecourt and a propylon in front of it in the east, where the grand avenue ended.
The Roman temple for Asklepios Soter or Zeus Soter Asklepios was located south of the propylon and thus on the edge of the courtyard area. The building is a smaller replica of the Pantheon in Rome. On the south-east corner of the area there was a two-storey rotunda that was used for the spa. It was connected to the cult center of the complex, the sacred, radioactive spring, by an approximately 80-meter-long underground passage.
The south, west and north sides of the courtyard were lined with columned halls, to the north of the north hall and in its western area was a Roman theater, which with its 29 rows of marble seats could hold around 3,500 spectators.
Source: Wikipedia