nibblot
Recruit

I think the problem with castles is caused by the party system. Currently, the logical way to play is just run around with a full party as your 'army'. There are lots of ways to make money trivial, so you can always afford to do that.
I would remove the limits to party size completely, and just make it ruinously expensive to move a large party around. Now castles are very valuable - that's where your party/army is stationed, with costs drastically reduced. They're also key strategic goals, because if you just wander past one you know there's a good chance a large force can sally out to attack or harass you. And because you're bleeding money the whole time, you can't afford to have your campaign constantly derailed like that. To make this playable, I would allow any noble to store troops at any friendly castle.
So: enemy decides they want to take a city. There are two castles in the general area between them and the city. They raise an army of 1500 (I'm imagining in this system each lord would be responsible for the upkeep of each party in the army, and the army commander is paying for his party and an 'army premium' to keep the logistics working). Every day this army is in the field, that kingdom is losing money, so do they take the time to take the castles or not? If not - a friendly lord can raise an army between their party and the two castles and attack while they've sieged the city. He can afford this because he only has to pay for this army for 2-3 days maybe. If the enemy does take the castles, it's expensive and difficult maybe, but now your kingdom has to march an army much further to counter attack.
I would remove the limits to party size completely, and just make it ruinously expensive to move a large party around. Now castles are very valuable - that's where your party/army is stationed, with costs drastically reduced. They're also key strategic goals, because if you just wander past one you know there's a good chance a large force can sally out to attack or harass you. And because you're bleeding money the whole time, you can't afford to have your campaign constantly derailed like that. To make this playable, I would allow any noble to store troops at any friendly castle.
So: enemy decides they want to take a city. There are two castles in the general area between them and the city. They raise an army of 1500 (I'm imagining in this system each lord would be responsible for the upkeep of each party in the army, and the army commander is paying for his party and an 'army premium' to keep the logistics working). Every day this army is in the field, that kingdom is losing money, so do they take the time to take the castles or not? If not - a friendly lord can raise an army between their party and the two castles and attack while they've sieged the city. He can afford this because he only has to pay for this army for 2-3 days maybe. If the enemy does take the castles, it's expensive and difficult maybe, but now your kingdom has to march an army much further to counter attack.