Monster islands are areas where you build buildings to keep your monsters busy and happy. In each game, there will be different types of an island. There will also be miniature dungeons which you must break through with the help for dragons or other special creatures such as Mermion's Eyrie. You need to multiply power or use powerful skills on these dungeons to unlock them so you can enter.
Some monsters will be playable in this game. You can collect these by gaining experience through the monster fighting maps, and then it unlocks them for you to use on each map. For example: King Ghoblin is a cheap and simple one-time purchase (that takes 30 hours of gameplay).