The buttressing of the walls would be tested in AD 256 when Shapur I besieged the city. True to the fears of the defenders, Shapur set his engineers to undermine what archaeologists called Tower 19, two towers north of the Palmyrene Gate. When the Romans became aware of the threat, they dug a countermine with the aim of meeting the Persian effort and attack them before they could finish their work. The Persians had already dug complex galleries along the wall by the time the Roman countermine reached them. They managed to fight off the Roman attack, and when the city defenders noticed the flight of soldiers from the countermine, it was quickly sealed. The wounded and stragglers were trapped inside, where they died. (It was the coins found with these Roman soldiers that dated the siege to AD 256.) The countermine was successful, for the Persians abandoned their operations at Tower 19.