I think it would be unfair if shields could withstand horses any better than an other weapon. If shields would help, just holding block with a weapon should work too. Anyway, I think horses bumping people down is great as it is. However, I think the damage from horse bumping should be at least divided by 5. It's not good for the playability/fun that cavalry can kill most players in 2-3 bumps. If someone is going to play the "realism" card here, it's perfectly realistic that getting run over by a horse hurts, but it hurts at least as much for the horse.