Another Tower Defence game, but very simple and it retains its playability. Did I mention it's multiplayer?
The game originated in Germany and the player-base is largely German/ European. The mods expect a level of ediquitte, and are quite strict with bans; really great for keeping noobs out. There is still sort of a noob problem, but the administers really strive for a good environment.
The flag of your country appears next to your username.
The game has a rating system, and some people take it as seriously and competitively as chess, particularly people from countries that feel the need to prove themselves, which is sort of a "thing" that I'm still trying to wrap my head around. Not too many folks though, I only play for fun.
You can play immediately at CreepTD.com without download
http://media.desura.com/cache/images/games/1/18/17168/thumb_940x3000/CreepTD-Game-Bubbles.jpg
