Just some basic information.
Tips to Basics
Castle levels require less population, but upon conversion to a town, the town is over populated already with no time to keep up with building progression.
Castles are for outrageous growth provinces if you get them early enough to convert them and for provinces which just will not grow to get them at least a set of walls. They can also be used to secure a general pathway, but understand, castles do not convert well to settlements without an uproar rapidly getting out of control.
Castles have no markets, but castles can without hesitation be fully upgraded in farm level as long as never converted to a city.
Castles are as well obvious military in their recruitment options and those same soldiers can only be retrained to full strength if in a castle able to build them.
Taking a castle you can always get away with simply sacking it. Taking a settlement of the same religion you can get away with sacking except for the major major settlements which need taking down a notch to give time to build enough support for happiness/control. Opposing religion settlements, slaughter away anything above tier two, but tier one and two slaughtering usually sets the settlement back too far in population to be of use for a bit.
Settlements are populations gone wild once they are past or into third tier. Some, by design, are instant as soon as the game starts off the scale in growth. In order to limit this, you must plan early taking commerce buildings not past markets and farms not past level two. This cuts what works out to be growth and squalor though it is not quite so obvious in M2TW. Dropping those building progressions also gives more time on both ends to getting your religion and happiness or control support building up along with military.
On the field of battle, cavalry are tanks. They sit still and they get slaughtered. They go into spears they get slaughtered. Cavalry for for opposing swards and ranged preferably by attacking in the sides or rear. Cavalry run out of stamina quickly running uphill especially and it last longer running down hill. The proper use of cavalry is never to attack the unit you are colliding with, but rather always clicking beyond the unit you want damaged click after click forcing them to run completely through another unit. Turn them slowly giving them a moment and do it again.
My advice is to turn any province with under 2000 population into a settlement if it doesn’t already have a decent growth rate (above 2% at least). towns have more building options that accelerate growth and you need 4500 pop for a fortress level castle, and 9K for a citadel. Keep your taxes low in the provinces you want to grow rapidly and plop a chivalrous governor there because chivalry also adds .5% growth per point. I mention fortresses because they train better troops. don’t waste your money on inferior troops which you wont need in early game anyway. Also don’t turn a settlement into a city (6000) pop because you cant go back to castle chain once you build stone walled city.
In the first few years grab a few neutral territories and try not to make too many enemies. Don’t over expand your realm where you cant defend it all. You can get away with a ratio of 10 cities to 2 castles if you supplement early with mercenaries.
As for Combat just look for enemies you can take while they are travelling, but before they join another force. Not every battle needs to be full stacks in the early periods, you cant afford that anyway.
IMO cavalry are your killers, followed by archers. Infantry are only there to engage and pin enemies so your other two can wipe them out. Only attack those you think you can defeat, and defend against those you cant by using terrain, choke points, artillery, and anything else you can think of.
If you have an ally, and you notice he has a force adjacent to an army that is your enemy, if you engage the enemy even with 1 unit, your ally more often then not will aid you and declare war. Then your ally as well as your enemy are depleted while you sit and watch.
Lastly play at whatever level your comfortable with and go harder when your ready.
One more thing in regards to charging. If your cavalry unit has a lance as they are charging forward watch to see if they lower their lances and sound the bugle. If they don’t it results in a sloppy engagement and you will suffer losses. If you do it right you will watch the enemy unit literally melt away in seconds. then retreat back a bit before they can recover against you and cycle charge again. It works best against rear and flank obviously, and best against stationary opponents. Try to avoid frontal charges against spear troops above militia grade, but otherwise one unit of knights can clear the field of any number of enemy footmen.