Excellent idea Sir Knight, but pray-tell, how shall the king watch over the battlefield and command, without something besides a spectate ability, or binoculars? You say it would be a rather long battlefield, and that would be difficult to survey and command over with just the zoom... unless...
A keep should be implemented, at each end of the battlefield. The King can heal while inside his keep,(or not) and it provides him protection from the outside, but only 10 of his men(not including himself) can be inside the keep at a time, this would generally include both escorts, and 8 elite infantry such as huscarls or sergeants.
My idea for the keep's design is this, for each faction(or map playable with this mode) a different keep is placed on the battlefield.
For the Rhodoks, the giant round keep of Grunwalder Castle, for the Khergits, give them 4 short(about 15 feet) walls after their fashion(all sandy or whatever)with battlements, all in a square.
For the Nords, a tall wooden(relatively thin) watch-tower like keep, with stairs going up it to Ragnar(lol), with only the bottom floor of dirt, and one floor after the stairs, and finally the roof upon which the king stands, unless beleaguered by archers, at which time he would go the the floor under.
For the Swadians: A plain stone keep, like the model you have within the walls of the ruined fort map, only not hollow, and not surrounded by those walls.
And last but not least, the Vaegirs: octagonal walls with a stone foundation, and wood battlements on which the marksmen could go.
Of course you could just have two plain stone keeps regardless of faction or map.