Well, in a forces of Good (Human, Elves, Dwarves) VS forces of Evil (goblins and kobolds, although I doubt these 2 civs will collaborate easily) scenario, each civ can act accordingly.
Imagine a battle where the 5 civs are fighting (Good VS Evil scenario). The map is a mountain side with a goblin tower above and a cave below. A road lead to the cave, and the road is surrounded by a deep forest.
The defenders (goblins and kobolds) will, of course, defend their tower / cave. The humans could attack quickly by riding down the road with their horse, the kobold have prepared traps and spikes and are ready to survive the onslaught... when suddenly a death rain of arrows come from the forest ! The kobold defense is crippled, and the situation is clearer for the human warrior. The kobold ask for help from the goblins, but they don't come. Why ? Because the dwarves (that attacked from behind, by the mountain) have breached the tower defenses ! Then a detachment of humans will climb the mountain to flank the goblins. Battle won.
Colored map (D = Dwarves, U = Humans, E = Elves, k = kobold and g = goblins) :
^^^^___^^D^^^
^^^^||||^^^^^
^^^^|g||^^^^^
^^^^^^^^^^^^
^^^^*k**^^^^^
^^"'E"++++""E""""^^
"""'"""""++++"""""""""""
"""""""""+U++"""""""""""
"""""""""++++"""""""""""
It could work in reverse order. Let's say you're running your fortress like every other day. Suddenly, a goblin siege appear ! You lock your doors and order everyone inside. Provided you didn't hermetically sealed the fortress (raised bridges etc.), some kobolds could open the doors using their thief skills, allowing goblins to enter. They could also disarm the traps spotted on previous attacks. And when you're fighting goblins on the now open front gate, another squad of kobold bowmen will enter by another door, stealthily penetrate inside the living quarters.... and start slaughtering every dwarf they encounter, knowing that the fortress army is busy on the surface. Creepy.