Complex sentence: After some debate, a decision was made: the Castilians could not be allowed to siege Lisbon once again, since the city would undoubtedly fall, so the Portuguese would intercept the enemy in the vicinity of Leiria, near the village of Aljubarrota.
Split into simpler sentences: After some debate, a decision is made: the Castilians cannot be allowed to siege Lisbon once again, since the city would fall with no doubt. They decided to intercept the enemy in the vicinity of Leiria, near the village of Aljubarrota.