As much as i would love my medieval dream castle Ill stick to something more reasonable. I think a tower would be an excellent addition. They obviously date to ancient times and can encompass many building materials and shapes. It would be a nice change to build up instead of out all the time. Obviously towers should height limits but I think they would be a nice way to help reduce land need.
Maybe they could be a modular build. Say you build each level up to a certain number of levels. We could even limit height based on building materials.
With the addition of actual cliffs and height I think the one tower on the hill is a fun idea.