There is a lot of questions of how to upgrade your castles and cities floating around. It is not really THAT hard if you have the right man in charge of course.
But, it takes a while to shape your characters into something capable so i hope this little guide will help you a bit.
As you probably found out, chivalry is not a guaranteed population boost, at least at higher level. I've noticed that traits decreasing squalor (which usually also increase population grow) and law are vital to go forward.
OK, as the thread title says, you will find here how the trait system works and how you can expand the characters abilities (both military inclination and governors).
There is a lot of text so if you look for certain stuff, better use the search engine implemented into browser you use.
It really works and the best way to get the traits fast is to do the same stuff (repeat the task) with selected characters and rotate them.
All details can be found here:
Credits to original poster