I've never played long enough to see level four forts, so I cannot say that I've had bad experiences with them. Perhaps this is why I think they're perfectly dandy, in any case I do not appreciate the arguments levied against them.
Their inclusion is necessitated by gameplay concerns. Without forts and their obligatory lengthy sieges the game would be unbalanced in all sorts of ways. The player could abuse the AI, by distracting it with one half of his army while running around with the rest, capturing all of it's regions with ease. The AI's wars against itself would become chaotic flashes, as one country would simply overpower the other and proceed to dismantle it in a narrow space of time. This is just speculation, but undoubtedly the designers found good reason for designing the siege mechanics the way they did.
Other way around. When Castille fights France and loses, it's army loses in the first 5 months. Then for the next several years it's forts are sieged, it's war exhaustion is maxed out, it's getting no income, and it's stability goes to negative 3. Nearly a decade after the war was really lost Castille finally capitulates and gives France one province. It then takes another half decade of zero income and revolts everywhere to recover to the point where it can actually do something.
The AI was already fixed in recent patches not to let you put 1 man on all their forts while they siege one province with their whole army. The AI pulls their army back and wipes you out as soon as it sees you doing that. And in any case, Paradox hasn't exactly proven themselves to be masters of balancing the game, so let's not waste ourselves on appeals to authority here.
A good idea might be to reduce the siege time as war exhaustion accrues. 20+ exhaustion should practically give up as soon as your army arrives.
