Aqueduct

View from above on a portion of one of the seven aqueducts that brought water to Pergamum.  This portion is located north of the acropolis, and is about 800 ft. below it.   Note the arches that supported the inverted siphon that crossed this valley (compare the one at Aspendos here and here).  These arches are about 800 feet below the summit of the citadel.

Mark Wilson states that in "antiquity seven lines brought water into Pergamum" (p. 281)

Mark Wilson, Biblical Turkey — A Guide to the Jewish and Christian Sites of Asia Minor.  Istanbul: Yayinlari, 2010.