While in general I like the idea, I think your statement of " The idea of having one army for your realm would be laughable for anything except the smallest city states" is woefully optimistic. I would think that a very large portion of the realms in the game would not be able to support multiple special-purpose armies.
It's not that they are "special purpose" armies. What I'm saying is every realm has a mix of recruitment centers, and I think you might see a return to the sort of three-tiered armies a lot of realms used to have, except instead of it being tiered by the "ability" of the nobles in question, they would be tiered by the type of troops they recruited. So those who recruit from the elite centers go into the "elite" army with smaller units, then a middle army, then an army with as many cheap troops as you can get.
It's also a case of having to match whatever your enemy uses, and the terrain in your realm and your enemy's realm.
Exactly!
If your enemy is all open plains, and they BigBlob you through your one open rural, you will have no choice but to meet them there.
Or, you concede the rural regions and take their forests. Or, you lure them into a region they shouldn't be in. Or you let them hit the rurals and move around them to play a raiding game in their badlands. If their army is optimized for rurals, you can either play their game, or pick one of your own.
Your elite infantry army tailored to the mountain choke points will be next to useless on the open fields.
If you are heavily outnumbered, yes they will. But that's the whole point of this. You actually have to think about where you're going to be fighting, and adjust accordingly.
