Struggling with War

Hey everyone,

I'm fairly new to the game. I've been playing for a couple months, I understand cassus belli, and ive been through a few guides, but i cannot seem to get the hang of war. I'm hopeing that someone can show me what I'm missing.

The only dlc im using is Ruler Designer. I'm starting in 1066 as the king of the petty kingdom of mumu (people say Ireland is the place to learn and the king of mumu's difficulty bar is the lowest at the start of the game).

I dont go to war unless i have more troops than my enemy. I'll even wait until I've got the gold to hire mercenaries for months at a time. I'll usually win the first battle and often seige the castle in the territory but by the time i get to seige the city and church, I've lost too many men and the seiges wont happen so i have to give up. I've tried waiting 15 or 20 years to attack. I'll invest in my castle and my vassals' city and church to try and boost my tax and levies but as my numbers swell so do my neighbors'. I cannot seem to get a more than a 500 troop lead on them.

So what am I missing here? What are your prerequisites for war? Whats the best way to conquer territory?

Thanks for reading.