I first played a version of Tower Defense as a mod for Warcraft III. It’s a silly little game where you have creatures trying to escape, from some kind of prison, I believe, by running down the only path available. A twisty-turny affair. You need to keep them from escaping by any means available to you, and that usually means violence.
Bloons 2 is significantly less violent than its Warcraft cousin. It’s still the same basic concept, though. Balloons are trying to escape by running down a path… somehow. It’s your job to put up defenses that pop as many as possible, letting none escape. You get money for every balloon that you pop and bonuses for popping all the balloons in a wave. Which you can spend to upgrade your armaments. This is a Good Thing ™ since there will be more and more enemies with each wave.
I only managed to play this game for about 5 minutes or so. I made it to the 8th wave before I got tired of the silly sound effects and the tiny amount of money you get to upgrade your armaments with. I still like the War 3 mod much better, it just seemed like a better package to me. Still, give it a whirl if you have a few minutes you don’t mind never seeing again.