My first exposure to the tower defense genre was the bonus tower defense map included with either Warcraft III or the expansion. I'll give a short description of tower defense shortly but the quickest way to get a handle on the genre is to go play desktop tower defense. Seriously, go now. It will take you like five minutes. Then you can skip the next paragraph.
Tower Defense is a fairly self-explanatory concept. You are in charge of guarding some objective against waves of enemies, which I'm going to call creeps. You do this by building and upgrading towers. Towers shoot at creeps and, depending on the game, might be used as walls to guide creep progress. Every time you kill a creep, you get more money to build more towers. Every time a creep gets through, you take damage. Take enough damage and you lose.
It's a survival game. The creeps themselves get harder and harder to kill and/or more numerous with every wave. Typically different creeps will be resistant to some forms of damage and weak against others. This forces players to build different kinds of turrets and to generalize their builds.
Just this past year commercial tower defense games have started to hit the market. Defense Grid is a great example; a well playing, polished looking game available over at Greenhouse for $20. Lock's Quest, a recent DS game, has many aspects of tower defense gameplay. And, of course, there are a few iPhone implementations, Fieldrunners capturing the feel very well and Crystal Defenders cashing in on the FF Tactics license (don't get me wrong, it's still fun, but I paid for fieldrunners).
This rising tide of tower defense games is due in large part to the success of Desktop Tower Defense. RTS map mods and flash tower defense games have been around for years as I mentioned above, but Desktop Tower Defense refined the formula and somehow caught a wave of public opinion that got it played by a lot of game designers.
Tower defense is fun. Incredibly fun. When you think about it it would almost have to be -- otherwise it would still be some forgotten mod map on someone's hard drive. Instead, someone saw something fun they could do with an RTS engine and map editor and other people, finding it fun, made flash versions. These were refined until commercial game designers took notice, and wanted to make something fun. Tower defense isn't engineered to be entertainting, it's naturally selected.
What makes it fun? First, tower defense successfully captures the feeling of base building in an RTS without nearly as much micromanagement. Now, a lot of people like playing RTS's and like micro. I can find myself in that mood from time to time. But mostly I like building. I was a big SimCity fan and when I played the single player Starcraft campaigns, my favorite maps were the ones where you had to defense yourself from a neverending Zerg onslaught until the dropships showed up. Tower defense distills this aspect of play, letting the player stay focused on base building, allowing them to feel that groove instead of having to constantly switch attention between offense and defense.
The really addictive quality of Tower Defense is not, however, the building. Rather, it is the constant presence of the approaching apocalypse. Destruction is always around the corner, the tide is always rising. I've always loved this element when I find it in games -- one of my favorite examples being Fantasy Flight's remake of the classic Arkham Horror board game. This keeps the players constantly engaged. There is no downtime in tower defense. You are always on guard, always just short of the resources you need. It's a feeling similar to survival horror, a genre that some say has been on the wane for a while now. You are going to die, it's just a matter of when. I hear Gears of War 2 has a mode that captures this feeling as well.
Tower Defense recaptures that fear of death and the feeling of going against impossible odds that I love about hardcore games. Moreover, it is a genre in its infancy, a genuine mutation into something different from the nth generational shooters, rts's, etc. I'm excited about its future.