The walls only increase the range of defensive units by 5% per level of the wall (I guess ranged units only, not melees too, that I know of). If there are no traps or abatis or other defensive structures aside from towers, the fight just starts at the unmodified range of the longest range unit on the field. In this case it would be archers at 1200 feet. Cavalry travel 1500 feet per turn with HBR 10, enough to cross that distance in one turn. Though I still cannot explain why the cavalry's losses were so few, unless there is a huge technology gap between the two players. Even with no hero at all, 100k archers has enough firepower to kill about 20k cavalry per round with equal techs, and even if the tech difference was 0 to 10, it shouldn't be less than 13-14k killed the first round from the life and defense bonuses.