I have a hunch sieges will be improved in M2TW. They've come on in leaps and bounds since STW; I think that trend will continue.

RTW sieges can be rather fun - storming the walls, climbing up siege towers etc. I've started to enjoy them - particularly in BI, where you can have large, aggressive AI armies. MTWs sieges seemed lifeless by comparison and STWs were rather sad.

Requiring units to man castle defences - as M2TW apparently will - will go even further to enriching sieges. No more ghost archers, yay!

Having up to 3 rings of castle defence will make the largest sieges pretty epic. However, as Screwtype suggests, there may need to be more of an incentive to storm the castle rather than just starve it out - as storming a great castle does not sound easy.