Mapping and modeling are both done in Granny, which is a commercial modeling suite. Thus far nobody's really even tried to create or modify map geometry.
Since everything else in the game can be modified with freely-available tools or is pure lua, there is no SDK for moho engine games. You can change or even non-destructively hook any code you want that isn't in CPP (although you can even override/intercept calls to CPP functions in lua, if you want), which includes the map entity layouts - towers, portals, flags, etc.
I also don't know of any tutorials on it, but the map lua structure is fairly straightforward, and the only issue you'll run into is placement / coordinate translation, which will mostly a matter of trial and error. Good luck with that.
