I got another inspiration for a castle room system related to your castle based combat. This would be another gold sink you could add if you wanted. You could do things like build and upgrade training rooms, create growth gates based on minimum requirements for multiple rooms, sleeping quarters need to be upgraded, but so does the kitchen and training rooms, could be a shower room/ training event in addition to upgrading living quarters, building and upgrading barracks for ghoul labor, training grounds for thrall warriors, lumber mill, forge, and building a harem room or individual luxury rooms in order to create vamp queens/recruiters.
Now vampire queens have multiple functions, "recruiting", combat generals, various labor supervisors (construction, in house chores, resource gathering)