Dustole, I'll grant you that it's difficult to siege a one city realm, but certainly not impossible. After your initial hit, you can wait for the peasant militia to disappear a bit and then strike again. Also, the militia is all melee, so if you use your archers wisely in the next siege you can still win.
The defense of Oligarch costs about 6k gold on top of the huge income of the city. There aren't that many realms, especially one city realms, who could afford such a war. If you keep pounding Springdale, smartly, they won't stand a chance. Sirion made quite some mistakes in the sieges themselves as well and Oligarch gained a lot of nobles as well, which AQ doesn't have. Then again, Arnor doesn't have that many knights either.