After a room has been made with the regular flow of the map, is it possible to make that room the lower stack room? After checking other maps it seems those rooms have the mirror texture and are completed and set aside outside the main map. Just why does the courtyard in E1M1 need to be stacked anyway? Yeah, this one' s being kinda confusing
What? The E1M1 courtyard is stacked because a hallway goes over the entrance, the fences (i think) are maskwalls, and the tomb's doorway has an overhang. Just from the hallway, there would be much fukkupage.
Sectors can overlap (not exactly, they need to differ a little in x,y or both) in 2D. To avoid bugs, the floor and ceiling heights should be appropriate in 3D. Blow up the TNT barrel on the top to see why the sectors in E1M1 are stacked.