They each replace a settlement belonging to the same player, instead of being built from scratch. When a player upgrades a settlement to a city, they get the original settlement back. Although you may build as many magic cards as you wish during a turn, you may only play one - and you cannot play any card you have built in that turn. When you build a magic card, you should keep it face down in front of you - other players may know how many magic cards you have, but not what they are. For more details on the uses of magic cards, go to the magic card section.
Building is conducted one player at a time, going clockwise.
See house rules for our variation on this. The robber stops production at whatever tile he is present on when the dice are rolled. He is moved by either playing a knight card or by rolling a 7. In each case, the person whose turn it is chooses where he moves to. He must be moved to a different tile.