If we stop their teleportation magic, then the Skylance brings the fight to them in a way that they can't prevent. If there are any living members of the army, they can only march for 8-12 hours a day, which will be 24-36 miles, depending on the fatigue and subdual damage they're willing to suffer. The Skylance goes 96 miles per day with no effort from the troops, and it's got the defensive advantage of a fortress even on offensive campaigns. Compared to teleportation, it's slow as molasses, but compared to marching, carrying supplies, and camping at nights, it's lightning-quick.