World of Xeen II

I completed the game with 5 heroes and still don't understand how to build up the castle, not to mention the initiative bar at the bottom – who goes when, haha.

This isn't really a problem with Nival (as far as I know, Ubi just invested money, so there's no point in mentioning them) – it's your misunderstanding.
For some reason, most forum visitors understood the initiative bar, and they build up their castles without any problems.