In October 1580, Grey de Wilton with up to 4000 troops arrived at Smerwick and laid siege to the garrison. The invasion forces were geographically isolated on the tip of the narrow Dingle Peninsula, cut off by Mount Brandon on one side and the much larger English force on the other. They had no means of escape. In addition, the English had brought up heavy artillery by sea, which rapidly broke down the improvised defences of Dún an Óir.