How about: Whenever a Malakim unit in any non-city desert tile is attacked, it has a high (70-90%) chance of retreating if it would otherwise be defeated (just like offensive retreat abilities) Also give all Malakim units in desert an offensive retreat ability. (+50%) This bonus would allow mounted units to get close to 100% in retreat ability, showing cavalry(and camalry)'s ability to hit and run very well in the desert. I think this could be done with a free promotion that toggles when the unit moves. I don't think that deserts should be made impassible at all or have any restrictions on building roads. I also think that the Malakim need some production bonus, in addition to whatever combat or mobility bonuses they get. Without it they would need to become a nation of desert-border with lush insides or they would be outbuilt and out teched in no time at all. Whatever production bonus they get needs to be tied to deserts only, and not help them in non-desert citys.