FEATURES: 

• NOW WITH RETINA SUPPORT!
• 3D graphics let you play from an aerial view or zoom down to first person and get close to the action!
• 17 different enemies including the new Ghost Alien! 
• 9 standard weapons with 5 or more upgrades each (Plasmatron, Laser Cannon, Railgun, Mortar, Guided Missiles, Flak, ElectroSlow, ElectroShock, and ElectroBoost)
• An ULTIMATE weapon, the Nuke! 
• Purchasable special weapons: Flamethrower, Proximity Mine, and Flash Bait! 
• 44 included maps in diverse environments (forest, desert, snow)
• 28 purchasable expansion maps (Spooky, Moon, Lava, and Marsh Madness)
• Endless Mode: See how many waves you can withstand!
• Global leaderboards with daily, weekly, and monthly rankings
• 24 Game Center achievements with Friend Scores so you can compare scores with your friends on any map! 
• Replays let you learn new strategies or share your games with the world through Twitter, Facebook, or email.

Source: iTunes