The oldest section of Pergamon, the acropolis or upper city, sits on a steep naturally fortified ridge between two tributaries of the Caicus river. The Caicus valley provides access from Pergamon to the Aegean coast and the port town of Elaea in the west and the Persian Royal Road to the east.
The upper city, fortified in the 4th or 3rd century BCE, contains the 3rd century Sanctuary of Athena, the oldest cult center of the city as well as palace quarters, barracks, and arsenals. In the 2nd century BCE the 10,000 seat theater, the library adjacent to the Sanctuary of Athena, and the Great Altar of Zeus and Athena were added. In the 2nd century CE the monumental Trajaneum was erected on what must have been an earlier unknown cult center. From the upper agora a paved main street leads south and down-slope to the middle city.