Few things...
1) Taselak's Mobile CS - They have 13~15k at most now. Even if this siege fails, what are they going to do with their mobile armies? Refit? I don't think so. You guys are out of gold probably at this point. Run TOs? At most you will get two regions only to lose them in less than 2 days. So at this point, Taselak has been weakened enough and their mobile CS is no longer a threat.
2) Militia - You can't mobilize militia for two weeks. If a battle happens, the timer resets. So Taselak just has a lot of non-mobile units they can't really use. Reinforcing it will cost a lot again.
3) Wall - If you haven't noticed, it is expensive to rebuild walls on SI. Very expensive.
So to sum up, Taselak is incapable of doing all three things at once. Eventually, they will crumble. Also, Taselak's noble count seems to be going down pretty rapidly meaning after few battles, most Tasels will be gone and the city will probably be empty enough to be conquered.