The guys over at the Civalry mod have given us a preview of their system of fortifications and siege equipment. There is some really nice work here so make sure to check it out,

FORTIFICATIONS
Motte & Bailey
The first fortified constructions were simple wooden towers, usualy placed on a hill. Such a tower was called a "motte", and often a small village was built around it, including the house of the local landlord.
To offer further protection , a pallisade wall was constructed around this small village, called a "bailey". These primitive castles would often be improved and eventually evolved into magnificent fortifications.

Donjon
Over time the wooden motte was often replaced by a stone variant, called a "donjon". These were tiny castles on there own with a small food and weapon supply stored inside, allowing them to endure sieges for a couple of weeks. Enough to protect the landlord, his bodyguard and a few lucky peasant families against raiders and bandits.
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For all the information please make sure to head over to their preview thread at,
http://www.twcenter.net/forums/showthread.php?t=33076
and check out their sub-forum at,
http://www.twcenter.net/forums/forumdisplay.php?f=58

More soon I hope.