And yet, they did anyway. Why was it that the English were able to move thousands of men around the French countryside for months? Either French border fortifications weren't doing the job of stopping their supply caravans or the armies were able to subsist off local area forage. How were the frequent large-scale raids of the Iberian peninsula conducted? How did first the Arabs then the Seljuk Turks remain such a persistent threat to the interior of Eastern Roman Anatolia, even in the face of unreduced, uninvested border fortifications?
Why didn't any of these armies go hungry and fall apart?
Your claim. I deny it.
This is pretty irrelevant the point being made. Taking your claim at face value -- "Armies could not advance without reducing fortifications in their way which prevented them from re-supply..." -- then the French should've needed zero tactical competence. Starvation should have done all the work necessary. But the English didn't collapse due to starvation, in spite of being in the French interior for months.
Sure. But that's not the solution people are proposing in this thread, that local area forage be a consideration. It is all various combinations of flypaper mechanics for castles on armies, designed explicitly to create a frontline when that sort of thing was firmly post-medieval.
You are pointing at exceptions and claiming its the rule. Those cases were not only unusual throughout the world they were unusual in their own time as well. The Chevauchee's were unusual even for the hundred years war, most of it was the usual siege warfare. It's kinda been forgotten about since historians have something of an anti-french bias but the English had their asses kicked for most of the hundred years war. Within thirty years of Crecy the French had reduced the English to holding just a few ports. The Iberian raids you mention occurred at the same time, it was in fact part of a proxy war between the English and French. There were also some raids between the Kingdom of Granada and the Spanish but even so the majority of the reconquista was also based on the classical siege warfare.
And dear god why did you have to make things so easy on me by bringing up the 1177 campaign of Saladin? He got absolutely dunked on by king Baldwin precisely because his army was disorganized while foraging for supplies. Here's a quote from the wiki " Muslim historians considered Saladin's defeat to be so severe that it was only redeemed by his victory ten years later at the
Battle of Hattin in 1187". He didn't make the same mistake twice however, the purpose of the battle of the Horns of Hattin was to neutralize the effect of the castles by luring their garrisons out and annihilating them. Only once that was done he was able to operate freely. That's exactly the fate that the French should have dealt to the English at Crecy, which is kinda why it matters.
The Seljuks had early successes against the Byzantines with raiding but even then they spent most of their time very concerned with fortifications, both establishing their own and reducing those of their enemies to facilitate their own offensive actions. I doubt you would have used the Seljuks as an example if you realized that the Georgians, Crusaders, and Byzantines did use fortresses to protect themselves from the Seljuks successfully. Hell the Georgians absolutely crushed a Seljuk army which is likely what led to their downfall. I don't know why you bring up the Arab conquest, they attacked when the empire was weakened from war with the Persians from a direction that was not protected against, they took the fortified places easily they didn't bypass them. This is very similar to what happened in the previously mentioned Umayyad invasion thought that wasn't due to castles simply the Umayyad neglect of logistics.
Fortifications prevent re-supply, and without re-supply an army is of course forced to forage. Foraging operations cause them to spread out which leaves them at a severe disadvantage. This fact is why armies of the past rarely bypassed fortifications. The mere presence of the forts would usually prevent further progress until they were reduced. When they did bypass them they either left a containment detachment or just accepted the risk. Of those armies that did accept the risk they usually paid for it. This is a far cry from simply "prancing" on by as you alleged.
You can deny all you want but at the end of the day you are trying to challenge fundamental military principles held by military scholarship, so the burden of proof is on you to demonstrate that fortifications were ineffective. There actually are some examples but you'll find that its due to some clever stratagem that forts are bypassed without having to reduce them. They are still obstacles that must be overcome.
As a further aside, the idea of a front-line is hardly a post-medieval concept. Have you heard of Margrave's, Marquis', Marcher-lords? There were also a number of East Asian equivalents during their feudal era's. They mean Frontier lords and guess what they used to defend their frontiers with? Castles. For example the Earl of Pembroke was considered a Marcher Lord and they used Pembroke Castle as a base to defend against the Welsh. Given that the interior of England was rarely troubled by the Welsh from 1138 when the Earldom was established to 1415 when the last welsh rebellion ended I'd say it was rather effective.
And as for the game, multiple suggestions have been posted here not all of them are simply flypaper. Some are simply people suggesting that the AI prioritize places on its own border rather than across the map. See the AI targets places based on manpower over any other consideration and for one that breaks the fourth wall because the AI is effectively omniscient and knows what all the garrisons are and two makes for very bad gameplay as the AI will just siege and counter siege places with small garrisons and ignore everything else. I've also found that because of this its hard to actually defend a place because if you move your army into a place to defend it the AI just selects another target.