This brought a third approach to entering the city. A ramp was raised again attacking Tower 14, but, as it was being built and the garrison fought to stop the progress of the ramp, another mine was started near the ramp. Its purpose was not to cause a collapse of the wall — the buttress had been successful — but to pass under it and penetrate the city. This tunnel was built to allow the Persians four abreast to move through it. It eventually entered the city and pierced the inner embankment and, when the ramp was completed, Dura's end had come. As Persian troops charged up the ramp, their counterparts in the tunnel would have invaded the city with little opposition, as nearly all the defenders would have been on the wall attempting to repel the attack from the ramp. The few city survivors would have been marched off to Ctesiphon and there sold as slaves. The city was eventually abandoned.