I think that regardless of the difficulty level, the game mechanics change at least at 0AD when if the AI was paying the same amount of resources as you, they'll start paying less or if they were paying more, they'll start paying the same. I think King is probably about balanced though in terms of early game coats and bonuses, however I believe the AI still gets free trade routes, wonders and all that. Others will probably have specifics, this is mostly from memory!