French sappers started to undermine the walls and to damage them through underground explosions. Thus, on 27 May the Counter-Guard Barlaimont was damaged and then stormed by French troops. They were repulsed later, but managed to blow up the position. On 29 and 30 May the Spanish withdrew from further posts which had become indefensible. Meanwhile, the French miners continued working. On 31 May the defending troops were withdrawn to the main wall. Bastion Barlaimont was also in danger of being taken by an attack after the work by French miners.