I believe the AI factions need to learn some defensive maneuvers, like training an early city defender.
"Early city defenders" can't stand up to 100-attack armies of monsters! And if you train an army that can, you've killed your economy.
Frankly, it's not worth it to defend your starting city right off. Sure, there's a 20% chance it'll die, but if you don't have to expend resources defending it, you're at a significant economic advantage. I prefer to have a roaming sovereign killing monsters before they even get close.
The AI needs to be more intelligent about early military, yes. But it just can't fight the OP monsters off as they are now.
I think your starting town needs some innate defense. I shouldn't be able to just walk in and take it, but we also have to recognize that it's practically impossible to properly defend it and grow your economy as fast.