The regiments had been disappointed that, while their comrades were sharing in Wellington's pursuit of Massena, they were far away from the scene of conflict; and when Beresford would have fallen back, as it would have been prudent to do, they became so insubordinate that he gave way to their desire to meet the French; and so fought a battle where defeat would have upset all Wellington's plans for the campaign, and victory would have brought no advantages with it.
Like Inkerman, it was a soldiers' battle.

Beresford's dispositions were faulty in the extreme and, tactically, the day was lost before the fighting began.
The Spanish portion of the army did no real fighting and, in their confusion, involved the loss of nearly the whole of a British brigade; and it was only by the unconquerable valour of the remainder of the British force that victory was gained, against enormous odds, and that against some of the best troops of France.
Terence was in the habit of often going down and chatting with the French guard at the gate.

Their duties were tedious, and they were glad of a talk with this young British officer, who was the only prisoner in their keeping who spoke their language fluently; and from them he obtained what news they had of what was going on.

A fortnight later, he gathered that the British force on the Aqueda had been greatly weakened, that there was no intention of laying siege to Ciudad, and it was believed that Wellington's main body had marched south to join Beresford.
This was, indeed, the only operation left open to the British general.

Regnier's division of Marmont's army had joined Massena, and it would be impossible to besiege Ciudad while a force, greatly superior to his own, was within easy striking distance.