Need help with middle to end game!

[Doug-Thompson]

TYou can beat the AI with militia and 1st level castle troops if you need to.

Most player problems with money come from building too good of an army too soon. People upgrade military buildings to get those good units, too.

[Lamprey]

The one thing that made the greatest impact on my economy are farms. At first I was reluctant to build them as in RTW squalor could easily spiral out of control and make cities a burden, but in M2TW farms are nothing but good.

In M2TW squalor can NOT go over 80%. It's capped.

Let me repeat: there is a squalor cap! Which means that once you reach it, farms and increasing population is nothing but good for you as 80% unrest is easy to overcome with some churches/inns/etc. Over time, farms end up being a huge source of income, easily an additional 25%-30% of your total income if you develop them. That makes a big difference.

Build farms first, then roads/markets/ports. If you focus on the economy and keep your city:castle ration at 4:1 or better, you will be swimming in cash. You literally won't be able to spend it all.

[TevashSzat]

Although squalor does cap out, it is also good to note that in some cities, the population is simply too high too start out with and grows too fast. Constantinople, Jerusalem, Cairo, Antioch, Venice, and Milan come to mind. If you build lots of farms early in the game, you will not have all of the buidlings that increase public order which as a result will force you to keep a significant number of military units wasting away in the city

[Gratian]

Well, i would say troubles in mid-game occur due to screwed-up start.
If you lose few early battles (or even win them with Pyrrhic victories) then you will face slowing of your Empire expansion and BIG problems later (e.g. you have recruited a bunch of mercs and they have failed to take some city in order to loot it and cover the bankrupting budget). I have abandoned one of my campaigns because of that (Hungary h/h). Later you just do not have enough money to build up your trade, since all your neighbors wage war against you (because you failed to finish them off early) and all your money goes to recruiting everything you can get...
Just my IMHO, it has been so in my games for England, Spain, Hungary and HRE (especially for last one, since it was played on vh/vh ). Not sure if it is true for factions though...