Re: Invaders from Sub-continent
Well the horses definately did. 
Most invaders were invariably hunter gatherers only the British bothered bringing supplies with them… every other raider lived off the land.
I agree with Queer geography played an important part indeed, the only other route was the Eastern Jungles and tribes there were always gonna have the edge fighting on home turf so yeah for anyone invading out of the subcontinent there was not much need to do so but plenty of raiders had good grazing and plenty of good farmland to gain if they had the strength to take it.