There is no information about making a stronghold in the core books, although I understand there is a supplement which I wish I could afford. On the other hand, I seem to recal (very vaguely) that the 2e DMG had a few listings for castles and such. But it's been so long since I've used 2e that such might be a false memory.
In the absense of official rules, making stuff up is more or less fine.
The 3.0 (and I'm assuming 3.5) DMG mentions that towns of "Large" size and larger are able to support "a" fortress, so that's the official rule I'm going by. You are right when you say nothing is said about the fortress itself though.
Both countries will be scattered with population centers of varying sizes... LOTS of thorps, but only one metropolis... and a buch of towns of varying sizes in between. That will likely make populations larger than what people usually expect, but I want you guys to have room for mistakes... forgetting to guard the back gate of one city shouldn't cost you half your population, for example.
As for making your own stronghold... go ahead and have fun if you wish. If you don't want to design it yourself, I'll find a generic fortress design for you. And all fortresses will have a Large Town or larger associated with it, from which your supply train will originate and from whom you will recieve additional CR if your reputation increases.
Fortresses must be mundane in construction unless you want to buy magic for it using your funds. Your fortress has a standard staff that is paid for by simple taxes (aka, this is flavor that says you don't have to pay for them) but none of the members of this staff are fighting men. You can of course put your army inside your fortress, or part of your army, or whatever.
If you get TOO extravegant with your fortress I'll nix it, but that's a subjective thing. And again, if you don't WANT to bother with designing a fortress, you really don't have to. If you really WANT to, you may.
The size of your town will be linked to the number of units in your army. Don't ask for an exact chart, because it'll be based on a curve... largest army on each size will get the largest population center, smallest will get the smallest.
Pyrex already answered Jemal's question.
I'm thinking about starting the actual game come the beginning of next week. How does that sound to people?