In a previous blogpost I said that I'll be doing some enemies but I redesigned what we call the DevDoc (for normal games it's a 50 pages doc in which you find everything about the game, it is a sort of GameBible). Instead of having my fancy lists a bit everywhere I put all my ideas into bubbl.us. The interest of doing this is that now I can have an overall look of the game in just one time. I can also check what's done and what needs to be done. But this is only possible because we are only two looking at this "bubble draw", a big team needs more descriptions and guidelines to be kept on the same game idea. Anyway, the assets looks clear to me now. Here is how i'll proceed before creating them: have an idea of the chapter (design), make a prototype of the chapter (with the app Tiled), make it look good (art phase), add the details and the assets (polish). So you can see that the enemies that I was talking about previously are a non sense in this phase of the developement.
I have actually designed all the 12 chapters, and made 5 with Tiled that waits to be tested.