Victory!
Stapel's stack at Najran has been defeated, with about five nearly-dead units of mine still standing. I reinforced the city with a newly built catapult, as well as the last Crossbowman built by Reeniville, and the two units (crowwbos+longbow) I pulled out of Reeniville earlier. As you can see, there is an interesting development where Stapel has left his border city empty. Since I can see most of the surrounding area, it might be worth it to send a few units over once he has moved his into my territory, so he can't get them back in time.