This build starts at corperal, because range accuracy is the most important thing for a artillery build (same goes for crossbows btw)
start with a couple of Horse Archers. These will be your starting units because they have more than just range attack.
Get up to 16 HA's, these don't need to stay at apprentice, upgrade them when you feel you need 'm higher level. My experience is that you don't need them over adv.
when you've got 16 HA's, you can start investing in catapults.
remember you want to keep these apprentice, once you got most of 'm up to 100% range accuracy, you can start kicking HA's out for more catapults.
once you thing you have enough catapults
start leveling them. Leveling cata's can be difficult, you'll need SF's for that!
Another very usefull site is:
http://www.ifaqdb.com/knc
guides and other usefull tools are on there :)