This right here is a short little bit of advice for players who wonder where they should put their armies in M2TW.
First off, this guide assumes you have the cash to buy what you need. If you're good at managing an economy, you'll do fine.
Secondly, this guide does not cover how to use certain unit types, nor does it tell you how to set up an army. That is a matter of personal preference.
Offense:
Personally, I prefer to make it so that I can hit my opponent one castle or city at a time. Now, this means that my troops end up taking casualties, and I want to move on quickly to the next target. So I set up two full armies, usually identical. One does the hard work of besieging, taking the enemy settlement. Then the second army moves in, swaps out the conquering army's depleted units with fresh ones, and takes over occupying the castle while the veterans in the first army move on.
This way, I have a continuous assault on the enemy, while keeping my men refreshed. Note: THIS IS EXPENSIVE AS HELL. If you're not currently at war, it is best not to do this, because you won't be able to support your economy by sacking castles and cities.
Defense and Peacekeeping:
First off, Rebels. These little <expletive>s will spawn quite often, more so as unrest in a particular region grows. Left alone, they'll block trade, damage resources, and generally cause havoc. That's why you need to kill them quickly. Usually I have a 3/4 stack of mixed cavalry to deal with this. You should too.
First, build a fort, and garrison it with a peasant. It doesn't particularly matter where, so long as it is near roads. Then load it up with cavalry. A good amount of light with some heavies mixed in. Now make sure to build one of these mounted outposts in the right places so that no matter where rebels spawn, they will be killed the same turn.
Now, you will definitely come under attack in any campaign. Best thing to do is to be prepared for it. First, look at your borders. Are there any natural obstacles such as rivers or dense forest? If there are, place forts garrisoned with a peasant at the choke points in the frontiers. These are sacrifices, meant to be lost within a few turns, but they will buy you enough time to either raise an army or get an existing one into place. For example, during my campaign as the Holy Roman Empire, my western border is very easily secured. With Staufen blocking one bridge, a fort from Frankfurt blocking the other, and my allies on Antwerp, I was basically immune to attack from the French.
If you lack natural choke points, you'll need to either avoid war on those fronts altogether, or build a large army for defensive purposes. These should be well-trained and upgraded, as they'll be subjected to lots of attacks. Place a family member in them so you can build forts and towers.