Originally Posted by
SoulGamesInc
Princesses count as Agents, and there is an Agent limit depending on the number of Towns/City's you control. (I think that's what effects it) So if a Princess comes of age when you have no space for her, she will remain in the Family Tree and the game will pop-up with random marriage requests for her now and again. There is no way to move a Princess from the Family Tree onto the Campaign map once shes there, this is another Medieval 2 thing.
I could be missing some info on the hole Agent Limit thing, look it up on Google lol...