Given the disparity of forces, I don't think we're going to win this by beating up the orc horde en masse. We don't have enough people, even if both forces combined, without regard to the fate of the non-combatants.
As far as using urban warfare, the orcs are actually a lot better suited to it then the Dorinthians are. On foot, the main Dorinthian weapon is the longspear, and fighting indoors is going to be close quarters. Where the big orc axes are going to be ugly. If this fight degenerates into a dozen house to house small unit actions, the Dorinthians are going to get cut to small steaming bits, whether by attrition or just by being outmatched at close quarters.
That said, if we can get the horsemen from both forces out and mobile, it will give us a big edge. The footmen, with the reduced area of the inner defenses, should be able to hold them. If Cal Dorin and his horsemen break out, to hook up with Dartis, Silas, and Caramip, then things go back to favoring the Dorinthians.
The horse archers can move quickly from place to place, butchering and bolting, to use Mr. Churchill's phraseology, while the orcs are deployed to surround and assault the inner stronghold. Basically, rather then fighting the whole horde as a chunk, the horse archers can move around the outside edge of the orc forces, chewing up what they can, and then fading away at mounted speed when the orcs mass enough to fight them on even or superior terms.
If Barok can harass their main camp, the two threats together might be enough to make them draw off. Or we could get insanely lucky and the mounted column could strike at the main camp, and take out the leadership, using Barok's efforts to assist and distract.