Aornos (in Swat, Pakistan) was the site of Alexander the Great's last siege, "the climax to Alexander's career as the greatest besieger in history" according to Alexander's biographer Robin Lane Fox. The siege took place in the winter of 327–326 BCE. The site was satisfactorily identified with the modern mountain Pir-Sar in Swat, Pakistan by Sir Aurel Stein in 1926, and has been confirmed since by archaeologists. It offered the last threat to Alexander's supply line, which stretched, dangerously vulnerable, over the Hindu Kush back to Balkh, though Arrian credits Alexander's heroic desire to outdo his kinsman Heracles, who allegedly had proved unable to take the place Pir-Sar, which the Greeks called Aornis. The site lies north of Attock in Punjab, on a strongly reinforced mountain spur above the narrow gorges in a bend of the upper Indus River. It had a flat summit well supplied with natural springs and wide enough to grow crops: it could not be starved to submission. Neighboring tribesmen who surrendered to Alexander offered to lead him to the best point of access.
Ptolemy and Alexander's secretary Eumenes, whose account provided material for all later ones, reconnoitered and reinforced a neighboring spur to the west with a stockade and ditch. His signal fire to Alexander also alerted the defenders of Pir-Sar, and it took two days of skirmishing in the narrow ravines for Alexander to regroup. At the vulnerable north side leading to the fort, Alexander and his catapults were stopped by a deep ravine. To bring the siege engines within reach, an earthwork mound was constructed to bridge the ravine with carpentry, brush and earth. The first day's work brought the siege mound 50 m (60 yards) closer, but as the sides of the ravine fell away steeply below, progress rapidly slowed; nevertheless, at the end of the third day, a low hill connected to the nearest tip of Pir-Sar was within reach and was taken, after Alexander in the vanguard and his first force were repelled by boulders rolled down from above. Three days of drumbeats marked the defenders' celebration of the initial repulse, followed by a surprise retreat. Alexander hauled himself up the last rockface on a rope. Alexander cleared the summit, slaying some fugitives (Lane Fox), inflated by Arrian to a massacre, and erected altars to Athena Nike, Athena of Victory, traces of which were identified by Stein.
Alexander was now free to pursue his journey into Punjab, and his reputation for invincibility seemed to be established in India. The Battle of the Hydaspes River lay in the future.