The year 1369 came to an end, and 1370 began. In the latter year, while King Enrique was in Toro, he learned that Gomes Lourenço do Avelar and the forces which were with him in Ciudad Rodrigo were making great forays into the surrounding districts and inflicting immense losses and damage throughout the area, a region which had not declared for Portugal. The king was much aggrieved at this news and decided to go and surround Ciudad Rodrigo. He left Toro and pitched his camp outside the town, vigorously attacking it with siege engines.

Gomes Lourenço and the troops who were with him, as well as Martín López de Ciudad, who was the most noble knight of all, along with Pedro Merchán and others from the town who had declared for King Fernando, all defended so stoutly that the forces in the encampment were faced with a mighty task. When King Enrique realized that, even with siege engines, cannons and the power of his crossbowmen, his attacks could not defeat them, he ordered a mine to be dug. His men began to excavate close to the Monastery of Sampayo, which lies some distance from the town.

Gomes Lourenço discovered this from spies whom he had deployed beyond the walls. Thus, at the spot where he imagined that the mine would come, he demolished houses inside the town and ordered casks to be filled with earth and rocks, as well as a vast wooden tower with the lintels of the house doors heaped inside it, preparing himself for the damage that they [the walls] could undergo.

The troops outside completed their mine, placing wooden piles under a large section of the rampart. Once the day of their attack had been determined, they set fire to the mine and began to attack the town on four sides, in order to prevent those inside from knowing the location of the mine, as the assailants believed that in no way could the town's defenders withstand the force of their attack. The onslaught lasted for quite some time, with great displays of strength, one side defending and the other side seeking to force their way in; meanwhile, the wooden piles which held up the rampart burnt away, causing some eighteen fathoms of it to come crashing down, all in huge chunks falling upon each another.