Questions:
1) Are there different modes of play (eg. Campaign) as well as a freeform country-building?
There are two types of campaign, first is victory through conquest where you have to conquer atleast 70% of the (known) world, the second is victory through glorious achievement, where you accumulate points for doing various things depending on which nation you play. Besides those you can chose to play single battles and there's a few other options I think, but I've never given those too much attention. But a real free build mode is not available I'm afraid. (Just like there's no multiplayer campaign

)
2) When you go into battle, is it realtime where you control all your units and use tactics and strategy?
Two types of battle, first one is controlling the battle yourself, which is realtime, but with the option to pause the events to give orders and such. The second option is letting the computer calculate the results, the outcome then greatly depends on your general and what troops he has available.
And I have a question more that I'd like to know myself:
How do I make feudal foot knights? I simply cannot find the requirements for them anywhere, I've seen them used by rebels and in crusades, but I've never actually been able to build them myself.