[Dedas]

That is why you need many fortresses, that way you can mass the right kind of infantry where you most need it. You also have to remember that fortresses don't cost any upkeep, on the contrary, they actually make more money in accordance with their admin value. That is why a more expensive fortress pays of in the long round (it is also more durable and generates more resources generally).
Another thing, please take a look at the Ulmish infantry. What do you see? High protection, slow movement, low gold cost, high resource cost, castle defence bonuses. What does that tell you? Their main purpose is defending your castles and hunting grounds, not embarking on long crusades against enemy players. They can do that too of course, but you rather want to have blood summons, knights and black priests (higher movement) to do that. The rangers and Tiers will infiltrate the enemy while you march. When you conquered territory let your Black Priests and Black Lords preach as they both have the inquisitor bonus (extremely effective when you have 10 BP doing it). At the same time you build a fortress on that spot, a fortress will be defended by Halberdiers (castle defense bonus) you build there. Repeat.
This way every unit is useful in some way.