You can conquer other towns, or raid them, or burn them down. Those battles are waged in whatever environment the players built, and now it's time to see if the castle design you figured out is up to the task of withstanding an attack.
First question(s): Have you thought about in which way you want to skew the offense-defense balance?
Will the battle generally be in the favour of the defender (as it were in most of the medieval times)? Or will you also have leaps and changes in technology influencing the balance between offensive and defensive power for a short time (e.g. the invention of gun powder that rendered medieval castles useless).
The offense-defense balance can have a huge implication on what kind of setup of factions you get (e.g. large steamrolling or small defensive factions) and thereby in the end what kind of game experience players will have.
I don't know if what I am asking makes sense. There is a whole sub-field of International Relations Theory that is dedicated to the study of the offense-defense balance and countless of historical accounts of how the offense-defense balance played a part in shaping Europe, so it is rather difficult to sum up.
Anyway since you are making a "
complex social game about conflicts and decision" I would really recommend you to look up some of the social science literature on the subject.
I would recommend Norbert Elias' "Civilizing Process vol. II" (Über den Prozeß der Zivilisation) if you haven't read it already. This book seems to perfectly describe many of the aspects that the game is about in what I would call a historically "accurate" and game design ready way. Also the some of "historical essays of Otto von Hintze" are quite relevant to some of the game mechanics you described.
I know and completely agree with your philosophy that "
gameplay > realism" but in this case I think that real world accounts of how real people reacted to medieval culture, institutions, technology and such can prepare you for how contemporary computer gamers will react in the same situations.
Second question: What role will marriage play in the game?
Sorry for being such a complete pretentious nerd though... but I can't help myself