Yep, I think I got your idea. In Blizzard's Azeroth TD (prob not the actual name) your builders/heroes walk above the actual ground level, as if they are flying. I was simply imagining what I saw in blizzard's map, only there would be more then the two levels available. However, the two levels couldn't move between each other due to the 2nd lvl actually "flying" due to an art placement above ground level, or so I believe ( I haven't looked at that map in a while).
In regards to the units, I was wondering how you were planning on movign them between levels.
In regards to my doodads question, I was wondering whether you planned on giving the non-table (floor) levels the appearence of walking on a web, or if they were going to simply walk on an invisible barrier (like glass)