You will need a Kalypso account to get the game running. If I'd known this I would not have bought the game.
@Kalypso: You have lost my recommendation of this game to other people. I have been playing the non-steam version for ages and only wanted to have it available within Steam. I should be able to do that without setting up an account that was never needed for any others of my Patrician games.
I have Venice on my wishlist... but now I wont be buying the game, unless you remove the need for an external account.