Once this pact had been agreed and Alenquer was his, the Master left for Torres Vedras where João Fernandes Pacheco had already begun the siege. The Master had siege engines and cannons carried there so that they could fire upon the town. The man who held Torres Vedras for the King of Castile, Juan Duque, was a Castilian nobleman, well-accompanied with men-at-arms, foot soldiers and crossbowmen, quite enough to defend the place. The Master billeted his people in the area outside the walls where he could lodge them best; in the royal palace where Count Gonçalo and João Fernandes Pacheco were staying, a guard post was set up night and day, and another was placed on the other side facing the castle.

Torres Vedras has a fortress set on top of a beautiful hill which nature has created in such a regular shape that it was as if it had been contrived by human hand. The land within its boundary, close to the town and round about, comprises cornfields, vineyards and other crops, which at that time were stripped bare, owing to the war. The town's walls encircle the hill, on the highest part of which is the castle. So few people lived between the castle and the town that they are not worth mentioning. All the population lived outside the walls in an urban settlement with many fine houses in well-ordered streets at the foot of the hill.

The Master wanted very much to capture and keep this town, and Juan Duque was very keen to prevent him doing so. For this reason, there were several skirmishes between those outside and those inside about which it is not worth telling, except for the long preparations the Master made to take it. He ordered the digging of one huge mine not to mention other saps he also had dug; this was to come out in the square of the Church of Santa Maria which is inside the walls between the town and the castle.

This mine was wide and roomy so that three men-at-arms could comfortably walk along it abreast. It was started as secretly as possible in a very secluded tent well away from the town, to the extent that, not only did the townspeople not get wind of it at all, but also many in the camp knew nothing of what was being done there.