actually rahl one field batle and the wars would be over most of the times sieges were not cost effective and unleass some "cheating" appeared for the taking (like spies opening the gates) most of the times the only reason to besiege was to force a field batle win the day and present the terms

even when you managed after much sacrifice to take over a city you would need to spend fortunes bribing important people and expend alot of humanpower resources to keep the city under control that was one of the antigonid problems when they faced the romans they had too many garrisons scatered around places like athens and whatever

in medieval total war the rate of sucefull sieges is a disgrace in rl it was very rare for a castle to fall because of a direct attack or even a walled city it was starving and using the catapults to send dead animals into the place hoping that disease made them surrender before the disease caught your own men and you would be forçed to pull back for health reasons