Are you certain you have enough Power Score (the gauge bar on top of castle build screen) to place those items? If not try removing something from your castle first before placing those more powerful items
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.