I've been doing a some supplemental reading to build up a little knowledge on medieval siege warfare after having recently finished Runciman's three-volume history of the Crusades.
I've been trying to understand what castles are for.
I understand that castles are tactically strong fortifications in the context of the weapons technology of the pre-gunpowder era, just as the artillery fortification was tactically strong after gunpowder weapons were disseminated among western armies. But just as the individual “Vauban” fortress was only one element in a larger strategic system, then so also was the castle.
The Western-European style of warfare in the Middle Ages consisted largely of sieges – the taking and holding of fortified places, including towns and castles.
Holding ground was the means by which wars were won rather than by fighting battles. Fighting battles was a risky business that, were it to go ill for you, could lose you your field army. So siege warfare was the primary means of prosecuting a war, with raiding and devastation a secondary strategy that was practiced upon your enemies' territories as a kind of economic warfare.
Your own priority was to limit the damage raiders could do you and to attack their supply-lines. Additionally you might take your army to attack the raiders whilst they were dispersed.
The Role of Castles
*Firstly we could say that they were springboards to extend one's domination beyond the limits of one's own territory.
*To jeopardize an enemy's communications or economic arteries should you have a stronghold close enough by.
*To support an advance.
*Blockade an enemy position. Counter-castles could be built to invest or baffle an enemy fortified place.
*Castles could act as store-houses for both the garrison and even to support a field army operating in the area. They might serve as an armory or mobilization store for equipping the militia.
*Defensively castles served as refuges. A defeated army might shelter in one. Local peasants and their livestock might shelter there from raiders. If relief seemed unlikely, a garrison was likely to become demoralised. Think of the collapse of virtually the whole of the Kingdom of Jerusalem after the defeat at Hattin