In the picture on the left, as expected, I cannot lock down anything that would block movement near a door nor place a strongbox. However, I have the same issue in the picture on the right. It appears that a difference of 20 Z is ok for a strongbox (26 for upper floor, 6 for ground floor), but within 16 you cannot (26, 10 for lower on footstool). However, I can place a secure chest, and lock down a blocking item (small table) on the same spot.
