Soldato
I've got a few caravans making me some cash, small army of approx 80 and enjoying it so far. How will I know it's time to take a castle ? and what will I need ? cheers
The main quest I think help kicks along you making your own kingdom. It's time when you feel like it though, or you feel ready.
Personally, I usually like to be extremely prepared, and would probably make a full army of each of my companions, and a full army of my own. Except perhaps an engineering companion who I'd keep with me. Then build up the garrison like crazy, since it acts like a useful well for you to dip into if your own army get's wiped.
Trying to buy peace quickly from the faction you just attacked is probably not a bad idea either.