Tower defense. It just doesn't make sense to me. But then, nothing in video games do if you really looked hard enough. Especially if you're an indie.
Tower defense is a genre of video games that concerns putting towers to inhibit and delay advancing waves of enemies that usually travel along a path to a main base. Some innovators of the genre include multiple lanes of advancing waves, playing as the advancing army instead of the tower builder, flying enemies, random paths, random towers, and whatnot. It is almost a brother to real time strategy, howbeit without movable units.
However fun to play, though, it just doesn't make sense!
Why would an advancing army ignore the towers in the way? Why don't they just destroy them when they encounter it, like all good RTSes. Its like the creator is making fun of your intelligence of tactics and strategy. No one is dumb enough to do that!
But I guess that's what makes tower defense games a bit more popular than normal real-time strategy games. RTSes make you think about how to attack and defend from a smart AI. Thinking is hard work.
In contrast, tower defense has only one strategy for the enemy AI, advance blindly onto the battlefield to the main base, ignoring all defenses! If that was an RTS commander, he would get slaughtered quickly.
So in essence, tower defense games are replays of the same thing over and over again, a failed and foolhardy AI attack in an RTS.