Tower defense games have been growing in popularity at a rapid rate ever since the success of Flash Element Tower Defense and Desktop Tower Defense in 2007. Of course, the basic concept for these games goes back much further than that, bearing many similarities to old arcade games and mods made for StarCraft. Essentially, you have to destroy multiple waves of oncoming hostiles (creeps) by placing defensive towers on the map. These towers can typically be upgraded and in some games, used to force the enemy to take a longer path. Here are my favorite free browser-based tower defense games.
Desktop Tower Defense
Other games have now taken the idea a little further, but the famous "hand drawn" version of Desktop Tower Defense is still one of the best. As well as unique artwork and sound that suit the game very well, it has a good variety of creeps to keep things interesting. Rather than having the creeps follow a set path, you decide what route they take by placing towers. If you're new to tower defense games, Desktop Tower Defense is a great place to get started.
Bloons Tower Defense 3
Instead of killing creeps, Bloons Tower Defense has you popping ballons with dart-throwing monkeys and a variety of other unusual contraptions. The ballons are on a path, but there are certain objects, such as glue and tacks, which you can place directly in front of them when you get desperate. There are a variety of tracks to choose from and it's a colorful approach to tower defense suitable for all ages.
Where most tower defense games provide only one upgrade path for each type of tower, Onslaught sets itself apart by allowing you to upgrade the damage, range, and rate of fire of each tower individually. Towers will also start operating in combination with each other when they reach the maximum damage level. There are a large number of towers you can unlock as you progress and 8 different maps to play.
Flash Element Tower Defense
One of the earliest tower defense games done in Flash, Flash Element TD was modeled on the Element TD mod for Warcraft 3. Each wave of creeps is initiated individually, which gives you time to build between rounds, and are awarded 1 wood every 7 levels to spend on research. You also earn interest on the money you have left over when the round ends, so spend wisely!
Warzone Tower Defense
Warzone Tower Defense cranks the graphics up a notch and uses large, open maps that encourage maze building. It has a high-tech military theme with missile launchers and plasma cannons. Some tower defense games can be won, but in Warzone Tower Defense the enemy just keeps coming until your base succumbs to attack, so the objective is to see how long you can hang on. There are alternative upgrade options for some towers, and the game has a variety of maps to choose from.
Vector Tower Defense 2
Another game from David Scott, creator of Flash Element TD, Vector Tower Defense employs vector graphics for a very distinct look. To add to the strategy, all the vectiods (creeps) and towers are color-coded; towers of certain colors are more effective against vectiods of certain colors. Towers can also be directed to focus on close targets, strong targets, or weak targets. Vector TD 2 has several gameplay modes Vector TD doesn't have, as well as a different set of maps.
In Pirate Defense you build fortifications to keep swarms of pirates from sailing to the top of the screen. There is no set path for them so good tower placement is critical. Some towers in this game have trigger mechanisms which you configure using the "links" button, allowing you to set up traps for the enemy. Unlike many tower defense games, you can win Pirate Defense by surviving 50 waves of attackers, which is probably a little too easy, but you're sure to enjoy the Pirate Song at the end.