What kind of castle would you like to see for the Sylvan faction?

Well, if it's a Castle, I suggest that when entering the Castle, you could see elves (resting) walking around with children, and you could also build a school or something similar that would increase the Castle's population, i.e., replenish the army with new recruits! You could also build a building like a tourist camp, which would bring in additional income!
P.S. Even serious elves sometimes want to relax with a bottle of beer or wine, laugh, and sing elven songs about past exploits, etc.