Currently there is not really a good way to gain any experience for Engineering early game until you actually go start or join a siege for the first time. This makes it extremely difficult to take a castle and only possible if you are leading large army. It also makes a lot of sieges lack siege machinery because it takes too long to build them. Even if the castle is vulnerable and weak it's still very difficult to take a castle, and you have to play the opportunist and depend on luck. That isn't bad but I don't think it should be the only way to take a castle, but right now the player doesn't have much of an incentive to put skill points into Engineering because it's mostly in the late stages of the game.
Here's my suggestion: make clearing bandit hideouts a way of getting Engineering exp. Hideouts are pretty much like a makeshift castle/fort, so it could just get tweaked a little to be more like a fort with walls. Then the player has a mini siege on the fort and that teaches them the basics of sieges. Then they have a foundation for when they siege a real castle. There's already the waiting phase for attacking the hideouts so it could just be turned into a mini siege preparation and use basic siege equipment to help attack the hideout. It would also help with the issue that you can only bring a few troops to attack the hideout. This would give the player a rudimentary level skill of Engineering so that it could give the player a decent start on sieging castles.
Here's my suggestion: make clearing bandit hideouts a way of getting Engineering exp. Hideouts are pretty much like a makeshift castle/fort, so it could just get tweaked a little to be more like a fort with walls. Then the player has a mini siege on the fort and that teaches them the basics of sieges. Then they have a foundation for when they siege a real castle. There's already the waiting phase for attacking the hideouts so it could just be turned into a mini siege preparation and use basic siege equipment to help attack the hideout. It would also help with the issue that you can only bring a few troops to attack the hideout. This would give the player a rudimentary level skill of Engineering so that it could give the player a decent start on sieging castles.