The Curtains of Perge

 

Built in the late third century BC, the fortifications of Perge were revolutionary for their time. The curtains are of two structural types; a simple solid wall, and a wall with buttresses, arches, a mural gallery, and embrasures for arrow throwing devices. This illustration shows the latter type, from the interior and exterior. The wall provides two levels for defense; an upper wall-walk and a mural gallery. The former was wide enough to accommodate a three span catapult, but the mural gallery was considerably narrower: there was only room for archers. it is possible, however, that temporary wooden planking was placed between the arches during a siege, allowing additional arrow-firing devices to be mounted here.

Reference:
K. S. Nossov, "Greek Fortifications of Asia Minor 500-130 BC," Osprey Pub., Fortress Series, #90, Plate B, Pages 18-19.