I figure it's time we have an official Onslaught thread. At first glance, it's just like Shock Defense and all those other creep-killer games, but there's some very important differences:
1.) Infinite levels with exponential difficulty. The first level is 10 creeps with 10 life each. However, by level 500, the creeps have over 50,000,000,000 (yes, fifty billion) life.
2.) Crazy weapon combos. Build nuke flowers, BFG lasers, cluster landmines, black holes, and more!
3.) New weapons added all the time. Fusion, Sniper and Railgun are all new, and the Fusion synergizes with the Tazers!
4.) It's all math. You can calculate your damage, use mods to tweak your range, rate of fire, damage... it's possible to get 4.9x10^19 damage with a laserchain, if you can pull it off, but it's usually too much of a pain and too much of a cheat, but you can start dropping nukes that do twenty-five BILLION damage, with maxed Railguns +800% damage cleaning up the mess.
My current high score ever is 5.8 billion. Who can beat me? WHOOOOOOOOOOOOOOOO?
EDIT: Also, past level 500, it slows down your computer because of how much stuff is going on. I Have a seriously overpowered drafting computer that is currently trying to slog it uphill through mud because of this game. Be warn-ed!