Wellington renewed the allied advance into Spain in early 1812, besieging and capturing the border fortress town of Ciudad Rodrigo by assault on 19 January and opening up the northern invasion corridor from Portugal into Spain. This also allowed Wellington to proceed to move to capture the southern fortress town of Badajoz, which would prove to be one of the bloodiest siege assaults of the Napoleonic Wars. The town was stormed on 6 April, after a constant artillery barrage had breached the curtain wall in three places. Tenaciously defended, the final assault and the earlier skirmishes left the allies with some 4,800 casualties. These losses appalled Wellington who said of his troops in a letter, "I greatly hope that I shall never again be the instrument of putting them to such a test as that to which they were put last night." The victorious troops massacred 200–300 Spanish civilians.