Every material / weapon / interior has it’s own associated power score value that means “how strong this is”, you can see it in the top right corner of the inventory icon. If it’s tier 1 ballista that has power score of 11, so removing it won’t give you much space to place something else considering even a simple stone wall is 15 points. Try deconstructing your castle and bit more and then consider what you want to put in there with the points and limits in mind.