Sieges definately need to be revamped. There unfortunately is no way to make realistic seiges. In real life defenders would build counter towers, dig countermines, conduct sorties on the besiegers foragers along with all sorts of make shift additions to walls to withstand rocks. The besiegers would also counter by building forts or castles along known supply points to the city, catapulting diseased animals into the defenses and bribe the less loyal of the defenders to open the gates or allow their wall to be scaled at night.

As for the game some simple things need to be changed. Realism is almost impossible to achieve thanks to the turns being biyearly (sort of) when most sieges lasted weeks and sometimes months. Units NEED to use up supplies. Castles and and cities hold X amount of supplies that way a small garrison can last longer in a siege. And for the besiegers X amount of troops need to be used to fully invest the city (stop supplies). Too few troops besieging the city would still allow X amount of supplies in per turn (assuming there was a road way open to a friendly province). And then there'd be the same sort of system for cities with a port (if they are present in the game) where you'd need ships to blockade the port.

Doubt we'll anything like you guys or myself have suggested though.