Watchtowers, as they work now, make it so that whatever they can see you can see instantly. Seeing as we're dealing with 6 month timeframes this isn't a terrible thing. Watchtowers should simply make it so that the enemy can't sneak into your territory, giving you an option to do various different things once they are discovered by the watchtower to have invaded your domain. This would also translate onto the AI side of things so that you can't go about wandering through their empire unhindered, and would make the enemy coalesce their units into one area much more easily to withhold an invasion.