The idea behind is to place buildings on the ground where the AI would put their barracks or mercenaries camp. This prevents the AI from building theirs. The AI will be limited to only their starting troops, and when combined with horse mercenaries you are pretty much a god among them.

Why it works: the layout of each stronghold is set so they can't just put a camp in a random location within their stronghold. It would be a sin to ruin their castle aesthetics, we exploit that to make it their downfall.

To execute, you first need to know where the AI would put their camps. Since campaign mode always has the AI to build exactly where things should be, we start the game and observe where the layout of their camps. I find it easier to check where everything is with the flattened view (press the space bar). Remember that (good to save the game at this point as a reference point in case you need to check the location again), start a new game and let the fun begin.

As soon as the game starts, hit 'P' to pause the game. zoom to each of the enemy strongholds (find the AI that has their barracks nearest to their starting point - we will start with them). Now from here you have a few building choices: if you are not sure of the exact location where the AI puts their barracks, you can put down a large building like a mercenaries camp. The camp doubles as a spawning point for your horse archers which is part of the strategy (note you can only place one camp and one barracks. A new placement will destroy your existing camp/barracks). If you are confident on where their barracks will be, you can build a house or two on that location. For more advanced players, you can place a gallow (or a statue if you don't like punishment) directly over the land. Once done, hit the pause button again (also need to unpause when placing the building). If you are quick, your first building will be built before their lord spawns. Now move to the next stronghold using the same method.

My placement preference: 1 mercenaries camp, 2 houses, 3 gallows, and maybe a barracks if I need crossbow men. Save the game once you are happy with the placement (probably a good idea to not override the save I mentioned earlier - that should be your save for building reference).

You should also create a horse archer army. I usually go with 10 units.

Send them around the enemy's castle to start killing their archers/crossbows. I like to micro manage my horse archers so they don't get hit. Have them go zigzag in front of the enemies.

Now, there might be some annoying AI lords that will rush their troops out. In most cases you should be fine just defeating them with your horse archers or simply retreat. But in some cases the enemies that you triggered would actually go back and destroy your building. So to play it safe you should stay at a far away distance that they won't rush their troops out until you have clean off most of their units with your archers. Although from my playthrough only mr pig would destroy your buildings with his macemen.

Anyway just rush the AI with your horse archers and make sure your building won't get knocked down.

While your horse archers are working hard at the front line (or on their way to the front line), do some base management to create a sustainable economy - i.e. expand your stockpile, build a market, lay down your granary, 3 lumber jacks, a quarry with 2-3 ox tethers (expand to two quarries with 5-6 ox tethers when your economy becomes profitable), build 2 wheat farms and a mill (these are a must).
Set the tax to low to get some quick bucks and turn back to no tax when popularity drops to 60 (but you won't with my management scheme - read on ;) ).

Because you will be in the heat of battle for a while I recommend spending some money to purchase wood to build the necessities. Notice I don't even bother enclosing my walls because you don't need to when cheesing AI in this way. The AIs won't be able to attack you.

If you are short on cash, sell some stuff from your stockpile, you really just need wood so anything else can go.

After 3-4 mins you can start building 4 bakeries. with them you should be able to give your people extra food portion. Crank your tax up until your popularity gain is at net 0 (hopefully your popularity is still above 50 at this point). Sell as many stones as you need just save 20-30 stones for your trebuchets.
You can start branching from this management scheme to create your own economy. I normally build some stinking ponds near the enemy base for +2 fear effects. More fear = more efficiency.

Ideally, you should have a horse army for each of the AI to keep things busy. I put them into teams and have them patrol around the enemy bases. Takes some time to get used to but once you get there it will take 30 minutes to take down 5 tough AIs.

Make sure your archers are retreating when their lord comes out from the castle to attack you. I lost a good many archers because of losing track of my archers.

At some point you want to get 3-6 engineers in front of the enemy castle to start taking the place down. Also a good idea to chuck some cows to their base if they have a large number of infantry on the ground.

For tougher AIs that have heavy infantry, I'd buy some leather and crossbows early to create crossbow men. Have them stationed at an advantage point or something in case when the infantry gets out you can shoot them with crossbows.

Also sometimes you might get enemies (snake man and wise boi) with a barracks as well as a mercenaries camp, just build on these two sites then you'll be sweet.