Here's an excerpt from "A Witch Shall Be Born":
"That Smile was on the lips of Constantius at that very moment. He sat his horse among a cluster of his men -- thick-bodied Shemites with curled Blue-black beards and hooked noses; the low-swinging sun struck glints from their peaked helmets and the silvered scales of their coselets."
I haven't found a lot of descriptions of the armies of Shem, but I'll post more as I come across them. It seems that the Shemites are a cross between semitic nations and some mediterranean cultures from the bronze age. In my opinion, ancient Assyrians work really well for Shemites.
Although Assyrians used chariots in their cavalry, I don't believe there is any descriptions of the Shemites using chariots.