How did forts used to work and why are they worse now?
Early on every province had a mini-fort and you could siege them faster. There was no Zone of Control (ZOC) so a lot of wars involved beating the AI Doomstack, then running down the AI armies, and then carpet sieging the place (using a lot of covering regiments to stop the AI from building new troops).
The devs thought it would be better if the AI had time to regroup after a failed hit, so they made forts beefier and gave them ZOC so that in theory the AI could retreat past some fort and you could not follow. Unfortunately, rather than doing something simple (like forts act historically to slow the enemy, like a 4x transit multiplier, and harass them, like a 2x attrition multiplier) they opted for a magical "thou shalt not pass" setup. Then then hiked fort upkeep to make so you had to place forts in strategic locations.
The AI, has ever since had to deal with the problem of fort pathing. If the province they want to siege (for wargoal or whatever) is past a fort do they siege or walk further around? Well humans know it depends. You want to skip around the fort if you can do so by just getting MA through somebody else for an extra province or two. However, if you have to walk around the Black Sea it is likely faster (and cheaper) to just siege the first fort before going after the next.
Likewise, where to build forts is not obvious to the AI. AI forts will often leave gaps through which you can maneuver and they will often leave prime chokepoints (e.g. the passes in the Alps) open. Let alone doing better stuff like having your forts on the flat for early game Hordes or forting up some clutch straight landing. Instead the AI pretty much always has its starting forts and those it gets through conquest. Worse, the AI is terrible at upgrading them as technology rolls on so only the wealthiest places (e.g. Italy) have up-to-date forts.
Then we have all the idiocy with mothballing. Even in AI-AI wars all too often you have AIs drilling their units on the border with the forts unmanned. An AI declares war (maybe because it allied with somebody who then declared and promised land) and the AI has a stack-wipe from the morale penalty on drill and has their fort occupied. Worse still, armies will march to wherever the initial order specified, even if the now manned forts prevent reinforcements from following them in or if they will get locked after seizing their target province with no choice but to lay siege to a massive fort in the Siberian winter.
To whit, at game launch forts were simple (too simple to really slow anyone down), but they were implemented in a complicated fashion that AI sucks at parsing. This has offset a lot of AI improvements elsewhere. And AI algorithms have never kept up with the changes. The AI's pathing, strategic value weighting, and dynamic recalculation have never matched what the fort system actually does. Of course, with all the new AIs and all the new calcs for all the old AIs with every patch, it becomes that much harder to do anything about it.
As it stands, EUIV forts are an ahistorical abomination that are a giant nerf to the AI.
