The Spanish force is conducting a siege with fewer infantry than the British defenders. Losing its cavalry and most of its artillery therefore weakens its prospects beyond the current engagement. The expectation is that stripping away those supporting arms would make a later battle winnable even if this one were lost. A subsequent check finds one gun remaining in a six-pounder battery. The target choice is based on what the besiegers will need for their next action.
Notable Point · 1:39:37-1:41:09
Damaging a siege army's prospects by destroying its guns
Losses among Spanish cavalry and artillery could decide the next siege battle even if the immediate fight goes badly for Britain.