Practically the map creating would consist of the following stages:
1) Major waters, forrests, cities etc...
2) More etc...
3) Marking freely areas where you can dploy units and can't. The area can be located anywhere on the map and it can be of any shape.
4) Saving, testing, cursing :p
Yes, I know that this can be cheated by creating a scenario. But scenarios aren't very practical solution when two human players are involved. Unit positioning is a bit hard since scenario editor sees it all. You would need a 3rd neutral person to do the deployment. He'd probably do it all wrong and forget the careful instructions you gave. No thank you! :rolleyes:
And this problem emphasizes even more when playing online.