If you want to really mess with the opponents, take both Master Scouts and Earth Magic and judiciously use Raise Land to make everything into hills.
Put it all over your own and neutral territory on every tile of non-enemy ground.
It will reduce their movement considerably while not affecting your own.
You can limit their movement to a crawl with outposts (your territory), raise land, and caltrops.
Even if they do manage to conquer the outposts, they will still be stuck with raised land. The land won't prevent you from re-conquering the outposts.
This strategy is pretty mana intensive, so you will probably not want to have a Mage sovereign if you do this.