Start by placing down crypt flooring which you'll get from completing the throne of command quest, it's seriously important for those servant mission bonuses. You'll want to set up your castle throne and servant coffins on this flooring, then build walls around them to create what's called a domination room - this keeps all your bonuses working together nicely. The castle heart needs attention too, since higher levels let you run more servant missions at once.

Got some space near the throne? Set up some stables after beating sir erwin, and don't forget prison cells once you've taken down vincent the frostbringer. The stables help with mount bonuses while prison cells keep your servants well-fed with blood vials. Something that's pretty useful is keeping spare gear in chests right by the coffins, makes it way easier to prep servants for different missions.

Make sure everything stays within that domination room you've built, and keep an eye on those floor patterns staying consistent. The whole setup might take some time to get right but having it organized this way really pays off when you're running multiple servant missions later on.