You can enter the Mystery Game from the pop-up that appears when you load the game, or you can access it from the store. There are six items available to win, and you'll be able to click on a box of your choosing from a variety of colored, patterned boxes on conveyor belts in front of you. The items are split into three pairs based on rarity (Common, Uncommon or Rare), with one of the Uncommon items being a Whooshing Witches decoration, and a Common item being a Magic Fairy, as examples.
According to users on the CityVille forums, some of the other prizes available are a Potion Shop, Pixy Palace and Backwoods Blacksmith. That being the case, it looks like this Mystery Game contains items both Halloween and "Fairy / Woods" in theme, which is a bit odd.
Even with that being the case, there's also a goal that goes along with whatever prize you end up winning, so you really do need to use your free play as soon as possible. We'll bring you a complete guide to those new Halloween goals just as soon as we can.
Check out the rest of our Halloween 2011 coverage right here.
What did you win from the Halloween Monster Mystery Game? Do you like this new feature? Sound off in the comments.