Im not sure, but if you tell your men to hold position, I think they will push the siege engine for a bit but once it gets too far, they will want to move back to where you told everyone to wait. If you dont give your men any orders, most of them should stay at the back whilst a few move up to push the engine (3 either side) and they should replace each other as they get shot dead by archers.
Id recommend getting more money, and a higher honor rating. Honor makes honorable lords like you automatically (you dont need to grind quests for each lord), and those are the types you want joining your kingdom. You want to buy land in cities near to where you will start your empire. Whichever faction you take land from WILL attack you over and over to get it back, even if you thrash them and they sue for peace. They will rebuild, retrain, and come at you again. You will need hundreds of thousands of denars to pay for peace treaties to end wars quickly (sometimes you will be at war, and two other factions decide to dogpile in on you as well), and the only way to get that kind of money is from owning hundreds of acres of land.
Work on getting recognition as king from every faction leader, not doing so means you cannot form trade agreements or defensive pacts with them. Strong factions never seem to accept you even if you have 99+ RTR, however they will IF you are at war with a faction they are also at war with. Ive delcared war just to get that recognition, and made peace straight after.
Dont accept any random nobles that decide to defect to your cause. The majority will be bad personality types (quarrelsome, debauched etc) that have loyalty only to themselves and get on with noone. I periodically check what available lords want to join me, accept them, get my chancellor to explain their personality type, and then reload and pick only the martial type lords. They are decent and proactive in fights but are honorable.
Your kingdom needs time for its lords to get to know each other by holding feasts, at the start most lords wont know each other and wont be friends, so they wont help out in campaigns. You want peace as much as possible to allow lords to hold feasts and improve kingdom unity, picking honorable lords helps here loads.