The capture of Elvas and Fort Santa Luzia would also allow the Spanish Army to proceed down the Estremoz road to the Portuguese capital of Lisbon if they so chose.

In December 1658 a Spanish army commanded by D. Grandee Luis de Haro camped in the frontier of the Caia River with 14,000 infantry, 3,500 cavalry, and several pieces of artillery.

De Haro distributed his troops in trenches, giving orders to kill everyone that approached the city, and prepared his army to confront the relief force the Portuguese would inevitably send.

The Spanish bombarded the city, causing panic and casualties among the civil population, while disease claimed 300 lives a day inside the walls.

He organized recruitment efforts in Viseu and in the Madeira islands and united the garrisons of Borba, Juromenha, Campo Maior, Vila Viçosa, Monforte, and Arronches into a single army.

This victory was in no small part due to the personal initiative and leadership of General Ribafria, who was keen on crushing the Spanish cavalry while they were on the defensive.

Meanwhile, De Haro continued to conduct a pitched defensive battle centered on his static trench line, and failed to deal with the encircling Portuguese forces.

De Haro abandoned his strategy too late in the day, and the Spanish lines eventual broke, with thousands of soldiers fleeing in panic.

The rout was so total that the Spanish artillerymen were unable to spike (disable) their cannons before the Portuguese captured them, and Cantanhede came into possession of 22 assorted artillery pieces among other loot.