11k traps won't kill 70k archers. Chances are the enemy hero had high atk and yours didn't have enough. I lost 40k archers on the initial attack with a 150ish atk hero without the war horn on, in which case I would lose 10k or so, presumably to the traps and towers.

horseback riding would have little or no effect on that fight as he wasn't using machines or cavalry except to rainbow.