You're better off popping over to the Taleworlds' PW sub-forum in future, plenty of people there who can help you.
Anyway, what exactly do you mean by "build a bank"? By default, there isn't any "bank" structure in the game; most people use various props to create the illusion of a bank (or other architecture). Banking with NPCs, item persistence, etcetera, are features of extended server-side Persistent World scriptsets, and are not native functions of PW. If you would like to use those features, you will need to acquire a PW scriptset; Alera Gaming provides one, feel free to check out the link in my signature for further information. Once you have said scriptset, adding it to your map is quite straight-forward (just add the invisible_chest prop).
