Medievil Total War 2

[Heen06]

1)What are princess for? should i use them as opposed to diplomats?
2)Is it important that i make use of my merchants?
3)Are priests/cardinals important (and whats the difference between them)?
4)I have NO idea what i should be doing...as in, should i upgrade to castles, wooden castles, etc? the game is leading me to believe that if i do i cant build any of the old buildings i hadn't already built.(
should i build any buildings before going to a castle? or should i build anything before going to a city? can i only make cities and castles, or is there another upgradeable path too?)
5)Any other tips in general about the game, or about settings to disable for much better performance?
6)after i win, there is a column that shows some number after men lost and men remaining, and it has numbers and then parenthesis. Example: 939(359). what does this mean?
7)how can i reduce my military upkeep? its costing me tons.