Anti-TD is short for anti-tower defense. Tower defense is one of my favorite game types-all the fun of looking down on a base you're building, but generally without the pain of having to manage resources or even usually bother much with maintenance. Gunning down hordes of guys who can't shoot back seems mean, but not when they're endless waves headed relentlessly for your base or equivalent. So what would the opposite of tower defense consist of?
Why, sending out those endless hordes, of course.
It's a non-mazing type tower defense, which is for the better as a simple computer program would be abysmal at trying that.
But how do you balance the endless hordes for gameplay?
Well, essentially you need to pick and choose carefully what kinds of "creeps" you send out. You have limited funds, and if you don't have enough money to send out creeps, you lose. Also, you can't just sit and wait, because the enemy drops towers for free periodically, making the path progressively harder.
So how do you win? By spamming, albeit carefully and correctly rather than mindlessly.
Through some mechanism or calculation that I don't perfectly follow, as long as you have creeps on the path, you make money, meaning that if you've chosen the right type, they'll be self-replacing and probably net you more money than you spent. There are around a dozen distinct creep types which each have five versions (and you don't need to unlock them, they're all available at the start). You'll generally want fast, tough creeps, which stand a much better chance of surviving for long enough for you to make a game-ending swarm such as the ones above from their revenue. The most expensive creep, the Schnell 5.0 (schnell being German for "quick" or "quickly"), costs $3000 in the game's money, and moves at mind-boggling speed; it's possible, if you play your creeps right, to make a swarm of lesser, cheap creeps that, while it isn't able to help you win directly, can give you in excess of $400,000 (enough for over a hundred blindingly fast Schnell 5.0s-and yes, I've done this).
So, not a perfectly balanced game, perhaps-even the toughest level can't stop an utterly endless swarm of super-Schnells-but one that is fun for a while.