[jaif]

I have no idea how to do these things, but I would build an approximation using the tools at hand.

Mobile "forts" are battlefield-only, not something for recruiting purposes. I would simulate them something like this:

1) It's a leader.
2) Icon of a few wagons setup side-by-side (not a square, just one or threed sides)
3) 4 misc slots, no others.
4) standard
5) 10-20 units
6) basic "gun" attack (treat like a crossbow)
7) fear (that's what guns really did back then)
8) large size, high prot, high morale.
9) supply bonus (it's a wagon, it carries stuff).
10) (edit) duh, forgot - it can't move on the battlefield.

I think that would cover it. Make it a construction summons (5? 7?).

Again, don't get hung-up on the word "fort" - it was just a mobile barricade to give infantry a fighting chance vs knights. In the game, you could form a wall with them and put your archers behind them; that's close enough to the original idea.

-jeff