I think I really just don't understand how to play these.

So I've tried my hand at Medieval 2 and Warhammer 3 so far. And while I do like them, I just don't really get what I'm supposed to do in the early game.

Cuz pretty much every time around 10 or so turns in, an enemy will show up with a massive army that can just steamroll anything I could possibly throw at it. And I can't counter it because I lack the money and upgrades to put out anything on par.

So the only way I've been able to get past the early rounds is by cheating and basically giving myself infinite money so I can afford things. But of course that sucks a lot of the fun out of it.

So what am I actually supposed to be doing early on?

Edit: Jerusalem and Karl Franz are the factions I play respectively.