1. Don't build a huge army, start withdrawing your stacks out of your borders. and send them in the frontlines. Border garrison must be relative only to your neighbor. If you stop building they will stop building. ALLY with the faction of your immediate neighbor. For example, ally with the Hungarians since the HRE is your neighbor.

2. You must have an uninterrupted chain of ships connected to another AI port. So keep building ships until you have 2 ships per lane.

3. Most MTW factions can be summed up in 4 words: "Go through North Africa". Don't blame me if your campaigns get too easy though . But don't start until you have a steady flow of ships. Remember, for every province you take you must cover it with 2 ships (1. to prevent rebellion 2. to send reinforcements to the frontline 3. to prevent AI from invading by sea 4.continual framework for trade).

4. Keep pushing up to Constantinople. I always finish way before the Mongols arrive, I only met them once. Your Billmen or Feudal sergeants can handle the Horses. Take Kazar if you think you can defend it against the Horde. Here's what you do. One Billmen/Feudal Sergeants per Archer/Crossbowmen/Longbowmen). Put the archers in front push "hold" button. When the Cavalry arrives. shoot. When they get close retreat them behind the back. Keep shooting. Engange with Billmen/Spear when they start charging. Retreat archers that are out of ammo, then get reinforcement. When the AI is on a general retreat, use your horses. Which should be in the back or in a distant forest hiding. Remember though, once shooting starts march your horses slowly to the AI's back far enough so they aren't noticed ( do this if there is a forest where you can hid your horse). When the AI are all engaged that's when you get your horses closer, so when they retreat, they are caught.

A great horse formation is this. 2 1/2 - 3 1/2 rows deep, put them on wedge when flanking but not when chasing. Depends on what horse you use and their numbers. I prefer the fast Steppe Cav.
Make sure you don't chase 2 units with 1 horse, one will rally and flank you. Once they are on a general retreat, rearrange your main troops again. Archers front, Spear behind, Cavalry reserve at very back. On the extreme side you must have armor piercing shock troops such as Militia Sergeants/Billmen. When the Spear/Billmen enganges a horse use them to flank.

A Royal Knight general is Ideal. Put it at the very back (middle position). Repeat this again and again. When your spears get badly reduced retreat and get reinforcement. You may also use a catapult at the very back. But they are most useful during bridge defense and when you are facing a lot of infantry. Longbows run out of ammo real quick so I'm not keen on them despite armor piercing capability.

5. To maintain Cashflow. Trade. Capture enemy faction leaders for ransom. Don't build too much troops especially the expensive ones.
Here is a good balanced army:
Royal Knight general
1-2 hobilars.
6-7 Feudal Sergeants/Billmen.
3-4 Archers.
1-2 Militia Sergeants/Feudal MAA (shock troops).
2 Urban Militia/Woodsmen.

Longbowmen are worthless, so I don't recommend them. Regular archers kill a lot more and they last a lot longer. Of course you modify your army makeup depending on enemy and location but that's a great base army and will beat any AI army. You use the archers the same way as defending but march them forward shoot, hold formation. When the AI gets close march Billmen/FS in front of them. keep shooting. No need for expensive shock troops.

In addition, don't hire mercenaries, don't garrison provinces unless really rebellious ones like scotland and portugal etc. (See my other posts against rebellions). Be picky with titles, give it to former heirs and HIGH ACUMEN DECIMATED UNITS. So you have unit with only 2 soldiers but their acumen is high. Give them a title and garrison the unit in the province. Or if you find a high acumen unit but full, give most of their units to a decimated one. Then give them a title and leave them behind. Ideally, you leave behind a 10 or less soldiers as governor. When you get good on this, the AI will give them terrible vices. Keep a lookout and change accordingly.

One another note, DON'T attack on land before you eliminate all the AI ships of the faction you are going to attack ( Once you have full control of the seas). 1. It will cause rebellions 2. it will block trade.

6. Take Constantinople ( by going through Egyptians, Turks and Byzantines) and Kazar. Beat the horde and start taking on the the rebels and Novgorod. Take Norway and Sweden. Keep moving West. Your attacks must be coordinated though keeping in mind chokepoint provinces.

I got 6 million florins as the Turks during my second campaign (when MTW came out). All the provinces are as upgraded as time will permit. My taxes are normal and I took my time with the campaign, completely finishing around 1300. After that I just finish 2/3 but do it in only 100 years with at least a million florins. So I know these tactics work.

Enjoy the rest of your campaign and keep asking questions.