One of the most well-received games of last year wasn’t a blockbuster game like Halo 3 or Gears of War, but a free flash game called Desktop Tower Defense. It is free to play online and didn’t cost the developer, Paul Preece, $10 million to make like many big titles—yet this game has been played over 10 million times, and was covered by Attack of the Show, Game Informer and even the Wall Street Journal.

So what made Desktop Tower Defense such a popular and addictive game? In order to understand this, we need a brief history lesson on the subgenre of tower defense games. The most current iteration of tower defense really began with Warcraft 3—if you’ve heard of World of Warcraft, then you’re familiar with the franchise. Warcraft 3 is a real-time strategy game in the vein of the ever-popular StarCraft. The idea is that the player builds and controls fantasy armies with the intention of annihilating the other player.

Warcraft has a huge fan base, which contributed to heavy modification, or “mods,” of the title. Mods are like hacks people make to the game using the core game technology to create new game modes or change the way the game plays. This isn’t difficult since there is a lot of backstory, artwork, and game objects for game modifiers to work with. If you’ve ever seen your friend play Defense of the Ancients, then you’ve already seen one of the best examples of the projects of these unofficial game developers.

Tower defense (or TD for short) games like Element TD were incredibly popular. The highlights of this game were the buildings and the ability to strategically place them in order to destroy formations of monsters. Think the battle of Helms Deep in the Lord of the Rings movie—where the bad guys clash against the walls of the good guys—except someone forgot to close the main gate. To prevent the monsters from killing the villagers, they must be defeated before they get into the entrance. Archers must be placed, the burning oil must be readied, and the catapults must be positioned to defend the castle. Positioning is everything—tower defense is basically a fast paced puzzle game with the addictiveness of Tetris.

These concepts translated really well to flash design games like in Desktop Tower Defense. There are three significant gameplay characteristics in a game like this: the shape of the map, the economy, and most importantly, the towers. The game map is very much an essential part of the experience. Usually TD games have labyrinthine roads, but the exact shape affects the most efficient tower placement. Along with that, there is a simple economy—you get money for every monster you kill. Money either goes toward building towers or upgrading a player’s strengths. Generally the game awards interest for saving money, which rewards players for fiscal responsibility.

But it’s the towers that really differentiate the genre’s games from one another. Generally, each tower has a couple of different properties—a radius of attack, strength, and special effect. Special effects generally include freezing, poisoning, and area of effect or “splash damage.” A player can generally upgrade tower strength—the stronger the tower is, the more money must be invested in it.

While Desktop TD is not the most original of the tower defense games, the game has certain aspects that make it really popular. For one, it’s very fast-paced and quick, making it a perfect game for coffee or study breaks—just visit the Web site and start playing. Second, Desktop TD is special because it gives players the ability to create a maze with their towers. This also allows the player to control monster movement, which adds a lot of strategic depth. Without a good strategy, tower defense games get really difficult really quickly, though trying and retrying strategies is half the fun.

Along with Desktop TD, other popular free online tower defense games include Flash Element Tower Defense, which is a flash version of the Warcraft 3 tower defense game, forgoing graphics for more accessibility. Onslaught is another popular one because of the addition of attack combos. And for those who are willing to spend some cash, Master of Defense and Garden Defense are two highly polished tower defense games—the latter gives you the task of defending your mom’s garden with robotic garden gnomes. Take my advice—these are definitely worth your time.