136 turns and no watchtowers? Any particular reason you avoid them?
Apparently you only fought small garissons. AI retreats to the town square only when it has few units. Otherwise it only places 3-4 (usually elite) units tops in the town square, and does its best to hold the walls and the gatehouse.
Actually, you have to surround the square before ordering an attack, otherwise you'll suffer unreasonable casualties. Often times AI will do its best to prevent surrounding (send skirmishers to harass your groups while they are moving to positions around the settlement).
On an unrelated note. I'm pretty sure there is a typo in tatar unit names:
Tartar sounds cool, but
come on.
Also, am I supposed to have these units:
Are they even from SSHIP or from vanilla SS? Is ther a valid way to get them (maybe convert some specific city to a castle or somethin)?
In my game I got them around turn 50-60 by what I suppose was a bug. They were garissoned in a castle I bought from Kievan Rus.