I've downloaded a few tech tree's for MTW and still don't understand why I can't build certain things.

I'm playing as the Danes.

I have a castle and a dockyard, why can't I build Caravels?

I have only been able to build a metalsmith in one province. I'm guessing, but does this require metal in the tradable resources?

I have a keep (and castle) and boyer's workshop, why can't I build crossbowers?