Aornos

Aornos offered the last threat to Alexander's supply line, which stretched, dangerously vulnerable, over the Hindu Kush back to Balkh, though Arrian (although disbelieving himself of this story) credits Alexander's desire to outdo his kinsman Heracles, who allegedly had proved unable to take a fort that the Macedonians called Aornos (according to Arrian and Diodorus; Aornis according to Curtius; elsewhere Aornus): meaning "birdless" in Greek.

According to Arrian, the rock had a flat summit well-supplied with natural springs and wide enough to grow crops: it could not be starved into submission.

The geographer Aurel Stein suggested that Aornos was located on Pir Sar – a mountain spur above narrow gorges in a bend of the upper Indus River, just to the west of Thakot in the Pakistani Khyber Pakhtunkhwa province.

Ptolemy and Alexander's secretary Myllinas (rather than the famous Eumenes) reconnoitered and reinforced a neighboring spur to the west with a stockade and ditch.

To bring the siege engines within reach, an earthwork mound was constructed to bridge the ravine with carpentry, brush, and earth.