from reading, film, and video games this concept has eluded me a bit for quite a while now.
when i went to north wales, the ammount of castles was incredible, and the idea of the English being present in all places with giant stone castles was an obvious and clear symbol which must have been a much bigger statement in its era.
however, i see and read often of great fortresses which served as a military base in a way for an entire area. from what i gather, medieval kingdoms would build fortresses along borders. often in some cases not that many castles compared to the area they are seen to protect.
now what makes me confused is this, what is to stop an army from simply taking everything but the castle, and waiting till they come out of them?. how do a limited number of castles on a border in anyway stop an invasion simply pushing past them. armies are seen to hit them like brick walls, whereas they could walk right past them.
once cannons became an actual thing for fortresses, i can understand how cities had a fort which could simply blast the shite out of any army which took up residence there. but in the middle ages, we're looking at many castles which don't sit anywhere near cities, and simply had their own small settlements surrounding them. so whats to stop them ignoring the castle?
the answer is probably obvious, but i like to learn everything i can about anything, and i get irritated at holes in my knowledge.
Very obvious indeed. :)
Supply and communication lines and control thereof.
Assume an army bypasses a heavily fortified position that effectively allows easier control of a region via patrols and relatively safe position. That army is at risk of getting cut of from command and supply lines thanks to easily launch surveys from given fortification.
Wars are rarely fought in the short term and controlling the countryside near and around your exposed army is key to an unasailed march (an army on the move and stretched across an area is at its most vulnerable). Capturing that fort provides a very significant advantage to yourself as you require less forces in a given area as well as a secure retreat and supply waystation.
Forts also have associated resources that are generated by the surrounding lands and are sent to this fort as a form of homage or payment(feudal system), this homage would default to the army in control of the fort as well.
In short supply and communication.
Walk past them
Something that's very difficult to do in video games is a realistic depiction of supply lines. When producing a unit in most real time strategy games, you have a one time resource cost, and the unit is then able to operate completely independently for its entire lifespan. Real life armies are extremely different, as they go through incredible amounts of consumables.
To give you a bit of perspective, King Edward III had 7000 longbowmen in his army during the campaign that led to the battle of Crecy. Assuming that each one used an average of 6 arrows per day (it would vary among units based on what they were doing each day of the 46 day campaign), the army would use 42,000 arrows on a daily basis, all of which had to be replaced.
Every castle that you come across is a potential threat to your supply lines. If you choose to bypass one, it has a garrison that could simply seize all your wagons of consumables for your army. So if you didn't invest every castle you came across, you had no choice but to escort every shipment of supplies with a force that is at least equal to the size of the combined garrisons of all the castles you've bypassed, or you'll risk being completely cut off and made helpless
Every castle you bypass is a part of your army left behind. If you leave them unguarded, they will harrass your army from behind. You can't besiege a fort with 100 warriors inside with 100 warriors of your own (they can attack your camp in the middle of the night, while you can't), so they tie up a lot of troops.
I know a much later example of leaving pockets of enemy troops behind - the Soviet offensive of 1943-45. But Red Army had a lot of manpower back then, plus it was important to prevent the Nazis from digging in at their prepared defensive lines, so some cities were bypassed by the frontline troops and dealt with by divisions following them.