Here is the latest building descriptions from the official TW site,

Monastery
A monastery is a community as much as a building, a group of anchorites who have retreated from the world and it's temptations in an attempt to find god's peace
...

Cesspits
Cesspits improve, slightly, the level of cleanliness in a settlement by providing each dwelling with a pit for all their rubbish and ordure. These reeking holes have to be emptied from time to time...

For the full details make sure to head over to,
http://www.totalwar.com/community/rtwbi.htm (they show the unit in 3D)
and check out our forum discussion on Barbarian Invasion,
http://www.twcenter.net/forums/showthread.php?t=27930

More as information comes to hand.