The roof holes are the alleged method of dropping Zyklon B into the "gas chambers," indicating the "gas chambers" are on ground level. The "crematorium" in this particular building is directly adjacent to the "gas chamber" and not even secured by a solid door. In fact the doorway is completely open with no door fittings even indicating there ever was once a door there. That's some pretty poor engineering, from scientists who were in the middle of designing rockets capable of space travel. Just my opinion though.