Keyboard and Mouse: AWSD to move, Scroll to zoom. Drag the bar to make floors visible/invisible.
Touchscreen: Two-finger touch to move, supports pinch-and-zoom. Drag the bar to make floors visible/invisible.
1) I had to create the transparent images for each floor manually. In the future, I would use Photoshop or similar software, instead of kludging it with Preview.
2) I forgot to insert an entire floor. Strahd veterans will probably know which one.
3) I discovered that the artwork, although very pretty, does not line up perfectly either with itself or with the outer shell image of the castle.