Chicken Invaders: Ultimate Omelette (or Chicken Invaders 4) is the fourth game in the series and is similar to Revenge of the Yolk. It has 120 waves across 12 or 13 levels. It includes weapons which can be upgraded through 20 power levels. The weapons from Revenge of the Yolk are available as 'unlockables'. Ultimate Omelette unlockables can be purchased individually by collecting keys, which are dropped from destroyed enemies during the course of the game. Key-dropping enemies develop a gold halo and must be killed quickly. The player must also watch the temperature gauge in the top-left, if the player overheats the spacecraft cannot shoot for a few seconds before returning to nominal performance. A new feature in this game is the capability of the player's spaceship to face in any direction depending on the level (it was always facing 'up' in previous games), and the camera can also zoom in or out depending on the wave (boss fights typically have the camera zoomed out to contain all the action). The game also comes in a Christmas Edition, an Easter Edition and a Thanksgiving Edition. https://en.wikipedia.org/wiki/Chicken_Invaders