that is an awesome drawing. I can see how you would be inspired to make this in 3D. I think I would have started working on this and said. . ."OMG, this is going to drive me to drinking"

On to some critiques.

The differences I immediately noticed from your model and the drawing is the squareness (is that a word?) of your model and the smooth flow of the drawing. Take a look at the doorways and stones on the drawing and then look at your model. While your model is well done, it just doesnt have the same "Other World" feel to it. The bricks and stones are random and your model they are very symmetrical. And the arches on your World balloon seem a bit off as they have some stone/bricks that the drawing does not.

Also some of the gear wheels just don't seem to be as "sturdy" as in the drawing. Notice how the main wheels/gears have thick support beams, your model could use some of that strength those bring to the image.

It is possible that once textured, your model will breath the same life the drawing does. You are doing a great job and this crit is in no way a dis. Hopefully I help you look with new eyes to make this a truely kick ass one of a kind.