This is an almost entirely functional map, designed as follows:
The area behind the player's back is occupied by sea (so whatever the player builds will be more exposed to attacks). The tree lines hinter building towers and moats (this will again increase vulnerability), and limit farmable area. When the plains are clear, the hills follow. Clearing the pass is only the first problem. Before invading the middle castles, the player has to deal with the sides as well. The castle in the back is just an extra fortress (that sends troops in the player's back while fighting for the hills).
An accidental feature is that the AI in the back sends its siege forces on the hills. If the map is played with allies on the plains, they might get some of it, if not, the troops become reinforcements to the middle castles for a while (it can still be frustrating).
The player's location is not suitable for the AI.
Feel free to modify the map to your needs or place some eyecandy if you wish.
I would also be grateful for other "Impossible Mission" maps, if anyone feels like making one.
I know Brass Dragon has said "I would also be grateful for other "Impossible Mission" maps, if anyone feels like making one."
Sadly, comments on a download page are restricted to being about this map itself.
However, I would encourage you two, Brass Dragon and Krarilotus (or one of you) to start a maproom thread on the topic of "Impossible maps".
That is just the kind of thing members like to discuss. If you start such a thread then you could add comment here to direct members to the thread in the Maproom.