In order to unlock Lego City, you need to get 60 Gold Bricks. As soon as you have them go outside the Cantina and assemble the bouncing Gold Blocks. Walk through the door that "magically" opens and you will enter Lego City. I have not been able to beat this yet and I think the key to beating Lego City has something to do with the "enchanted" house that spins away when you get near it.