In a tower defense game, you begin with a simple gameplay area that has an entry point and an exit. Enemies appear in waves of increasing difficulty and proceed towards the exit. Your goal is to defend your tower (or other item depending on the theme) and destroy the enemies before they reach the exit. Any enemy that reaches the exit before you destroy it reduces the number of lives that you have. Most tower defense games allow you to upgrade your towers over time to make them more powerful and effective against certain type of enemies.

Overall, the concept is simple and infinitely customizable. Developers can wrap the same basic idea in different themes, add a back story and some marketing. And, hopefully, have the next great iPhone game! In fact, many have tried. Here are a few that have actually succeeded — a list of continually popular tower defense games for iPhone. 

Popular even before the iPhone arrived, the tower defense game that most non-gamers are familiar with is Plants Versus Zombies. It’s candy-colored theme and cartoon cuteness are a big hit with casual players. In this game, all that stands between your house and a horde of zombies is the yard. Keep your home base safe by building up an army of plants. Let them across and your brains are toast. (download PvZ for iPhone)

Bloons Tower Defense is an incredibly popular franchise. The idea behind this game is that you must prevent streams of balloons from traveling all the way through various tracks and paths that are presented to you. The basic tower is a monkey that throws a dart, but there are many different variations on the theme, including towers that throw other things (liked cannonballs and sharp disks), as well as towers that launch bombs and missiles. There are even towers that will freeze the balloons to get them to pop. (download Bloons 5 for iPhone)

GeoDefense is a bit more abstract than Bloons. The towers and your enemies are glowing geometric shapes, vaguely reminiscent of things you might expect to see in an animated science fiction movie. The action is very fast, and the difficulty level can be very challenging. (download geoDefense)

GeoDefense Swarm, a game which received perfect scores from some if the top review sites, is the sequel to the critically acclaimed 2009 td game geoDefense. The second installment adds 30 more levels including one “endless” level, open field gameplay, new creeps, online leaderboard and stunning Retina Graphics. (download geoDefense Swarm)

Tower Defense: Lost Earth combines traditional tower defense game play with a compelling story (an alien discovery story) to drive the game from level to level. There are over 40 different maps to be played, nine different types of upgradable towers, ten different types of enemies and four special weapons. The game is very deep, and provides many hours of gameplay. (download TD Lost Earth)

EpicWar TD is a graphically intense tower defense game. Rather than cartoony or geometric graphics, Epic War TD portrays the playing field, towers and enemies in a very realistic manner. There are ten different levels of battle (with up to 60 waves of enemies per map, with 14 types of enemies), and different gameplay options for each map. (download EpicWar TD)

It’s become a cliché that no matter what you need, there’s an iPhone app for that. It’s probably true. Just remember that all work and no play makes iPhone a dull toy. So take a few minutes and play a game. These apps are great for quick pick-up-and-play gaming.