The Forsaken Fortress is a 5x5 sized Monster Residence available in CityVille. It is part of the Halloween Event (2011), when Monsters (Frankensteins, Ghosts, Mummies, Ogres, Witches and Zombies) rather than normal citizens moved into the city.
It costs 85 Cash and takes 6 Energy to build. Once built, it increases your city's population by 2,750 Monster Population. Rent can be collected every day and earns 494 Coins.