Monkeys popping balloons = Formula for Addiction

The tower defense genre ranks among the more addicting gaming options for the casual gamer. Unlike PC and Console games, all you need is an Internet connection to play, yet it has many of those qualities that make more complicated games addicting:

1. Short levels that give instant gratification. You get a little rush of glee every 3-5 minutes.
2. Something to protect. If an enemy gets past your defenses you gasp as your life force (or balloon quota) is depleted.
3. Steadily progressing difficultly. Causing a little stress ("Will I be able to hold the balloons off next round?") but making each victory that much more sweet ("Yay, my monkeys popped all the balloons!").
4. Simple Rules; Less simple strategy. To play you just need to drag and drop some monkeys; to win, you need to think ahead and save your money for more effective defenses and upgrades (monkeys with boomerangs!).