Couple good points brought up I'd like to make sure don't get lost.
1) Strategus needs more incentive for people to fight. There should be a disadvantage to being peaceful with your neighbors for too long. How you do that (stick and carrot both should be used) is up to devs. I do like that they are making crafting dependent on your fief, so that's some incentive to fight for fiefs. That's a carrot, I think one "stick" could be tax inefficiency. The more fiefs you have, the more inefficient you are at taxing.
2) There needs to be anti-siege equipment for defenders in battles. Attackers have siege shields, siege towers, and catapults. The defenders should have things they can throw/dump on enemies who are climbing up ladders. This may make castles impossibly hard to take, so it would need to be carefully balanced.
3) Plate is good for sieges, but pierce, and blunt weapons (bonus for crushthrough or knockdown) are able to effectively negate plate. Not to mention a lot of the pierce and blunt weapons are short, which is ideal for sieges. You guys have to remember this version of strategus has been going on for almost a year. And lots has changed since the beginning.
One last point that Slamz brought up, that I think is a major "pro-tip" and hopefully I'm only stating the obvious, not giving people battle tactics: Why don't more people sally out and attack on the open field?
Like slamz said, it's not like you don't see these 1000, or 2000 man armies coming. They are moving SLOW. They are generally siege armies, and thus have siege equipment. Most of the time they are not built for open field battles. If you sally out a cavalry brigade and attack them (along with other things you feel are beneficial to an open field battle) you're going to catch them off guard. The siege army most likely has ladders and siege equipment, not horses and pikes.
