City is divided into two classes; Tops and Commons. As a Tops, you have access to wealth, and, as such, can include the most expensive of cards in your Deck. If you're a Commons, you're only allowed to include one of each Monster in your Extra Deck. I suggest including plenty cards to revive your Ace Monsters, and make sure they stay alive. As usual, make sure your Deck follows the OCG Banlist and our own.
Another thing to note, you can't use cards that are included in an archetype that isn't your own, unless it also supports your archetype. Don't just fill out your Extra Deck; it should only contain cards that are related to your Deck or that your Deck is built around Summoning. This is all in effort to make Duels resemble those in the anime. If you have questions, don't hesitate to ask.
Another thing to note, you can't use cards that are included in an archetype that isn't your own, unless it also supports your archetype. Don't just fill out your Extra Deck; it should only contain cards that are related to your Deck or that your Deck is built around Summoning. This is all in effort to make Duels resemble those in the anime. If you have questions, don't hesitate to ask.