The map allows you to do what you're supposed to do. What more can I say?
The Arab force outnumbering you makes it a bit unrealistic. If I was the commander, I would not retreat with that force! I'd press it, and I'd probably win!
This is an original idea as far as I've seen, but the comments below drag this down.
Map Design: 1.5
The dunes were well done, I think. However, ask yourself this: Who would build a castle here? There's no water, no arable land, no resources... all is waste. Not to mention the fact that you clearly used the Beach terrain when Earth with Rocks, Driven Sand, and Earth would be suitable for at least the dry areas.
You told us what to do. No background, however, makes for a lower score.
It's a good idea. Work on the terrain, and the forces, and you will have it made.