This will be a representational model, not a diorama, so I'm not worried about fitting in stairs or being too picky about measuring movement. Instead of determining movement based on a figure's speed, I'll adjudicate it based on location and how long it takes to get there--so maybe one activation to get from one side of a level to the other, but two actions to move between floors. And yes, I consider that shaft to be open to all the levels, so you can have line of sight (and therefore shooting!) between characters on different levels. Now let's see if I ever follow through on this.