i can definitely confirm that Horse Archer;s will hit their own units, i had three units on a hill once just acting like normal archers (facing up a hill, twas in a city ok) and by the end of the battle i'd lost 25% of them thing is, noone came near them and he had no archer units. It was also a wooden palisade city so no archer tower fire to explain it either :(
ps:- i've said it before and i'll say it again. HA's rock in this game, they're everything the goldon horde from medieval shoulda been
pps:- ditto about the desert warfare/friendly fire/collatarol damage thing