I kind of worked the design out in my head beforehand, knowing how all the solidity would work, so it wasn't too hard. Actually the more I worked on it, the more graphics I realized I could delete because things were fitting together better than I expected and I didn't need as many graphics to get all the configurations I thought I needed. For example, notice where the slabs that run across the tops of the pillars intersect to form a junction. I thought I would need extra graphics to draw the intersection and I drew some, but it turned out that using the existing tiles would cause it to draw exactly correctly because of the way tiles overlapped. I did have to draw those slabs a number of times, though, because I kept getting one thing a pixel off, and the lines didn't exactly meet how I wanted them to. But mostly it worked out pretty well without a lot of trial and error.
Try out the map editor and see if you get an idea how the tiles work. They're kind of defined as columns of frames, and in some cases, pieces that would be hidden by other foreground are left out, and the tile is just a bit way up high, and only the solidity affects the area where you click. Oh, also, you can change the "ShowFloors" counter to 1 to see the actual shape of the solidity of the floors.
Did you figure out how to get the keys to unlock the door and the chests, and pick up the content of the chests?
The next thing I was thinking about doing was making ramps to allow the player to climb up to the top of the walls, where it would be able to walk on all the wall tops (by deactivating the main layer sprite and activating a foreground layer player sprite, where solidity for walking on the tops of the walls is defined). But I didn't get around to that.