Aornos (modern 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 BC. (The site has been identified as being near the Pir-Sar mountain in Swat by Aurel Stein in 1926, and has been confirmed by archaeologists.) It offered the last threat to Alexander's supply line, which stretched in a dangerously vulnerable manner, over the Hindu Kush back to Balkh. 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. Therefore, it could not be starved to submission. Neighbouring tribesmen who surrendered to Alexander offered to lead him to the best point of access.