I found the french campaign so easy, just because of your location and easy access to a few rebel towns to start.
Keys to victory:
Early game:
1 Secure all bordering rebel towns as fast as possible (gives you more villages, denies the AI)
2 Get trade rights with EVERYONE (you will take a small faction relation hit for trading with enemies of other factions but this wont matter in the long run)
3 Build up your roads markets ports and farms as high as you can as fast as you can in all your settlements. This should be first priority in any new settlement you conquer as well.
4 Plan your general game path now, which direction you want to head, who your first few nation targets will be.
5 Start preparations to join the first crusade when its called (usually called 20-30 turns in), this is an easy way to get an awesome general early on. (you should choose someone young, with decent stats, and preferably if you can someone in the direct line of succession)
6 If you are catholic do not declare war on any catholic state. If you wait long enough they will declare war on you and your reputation wont suffer as much.
Midgame:
1 Control the papacy if you are catholic. Choose one town and build up a church as high as you can get it then grab the theologians guild when its offered and start recruiting priests like crazy. After a certain point you will be pumping out priests with almost full piety and they will almost instantly become cardinals. Doing this makes sure the pope will ALWAYS like you. In the rare event that you do get excommunicated, you elect a new pope in a few years and you will be reconciled because the next pope will be french. (currenly in my france campaign with the exception of two preferiti i control all the cardinals)
2 Make sure that any towns bordering potentially hostile factions has at least a garrison of 7 stacks. You can use militia they work fine for defense. My default garrisons are 4 Ranged 3 melee (preferably with spears) You can throw one cav in there if you like, sometimes against stupid AI you can get your cav out in the field and flank their arty if they bring any.
3 Do not be afraid to let them siege you, do not fight in the field unless you have the clear advantage in either numbers or troop type. You have a massive advantage as a defending army. I have had a 5 stack of troops defend against a fullstack of troops just because i knew how to position them on defense.
4 Dont build up all castles, pick one or two "super" castles and build them up. Convert other castles to towns as you expand outward past them. Its best if you choose a costal area for a super castle and keep a fleet near by so you can quickly move troops about by boat instead of on foot.
5 Prioritize your targets, you will be at war with multiple factions but you can let them wash upon your walls on siege after siege as you deal with whoever you want. I was at war with HRE for years before i finaly took them out simply because i didnt need their territories and i had my sights set on england, spain, and moors.
Late game:
1 You control the papacy, call crusades as often as possible
2 invest in armories, they can make all the difference
3 do not neglect your markets and merchants. They will supplement the lost income from all the broken trade agreements.
4 If a settlement becomes too unruly send a general with low chivalry to the area, take the garrison out and let them rebel. Take the settlement back with the general and sack the populace. It should net you a decent amount of gold and the populace will now be in order.
5 Remeber to adjust your capitol to the center of your empire. it will help keep outlaying towns in order
6 Archers and heavy infantry will be your best friends. It looks tempting to have an army full of noble knights, but your bread and butter will be spear wielding infantry and archers. Remember this.
