Interesting little tower defense game...also bone-shatteringly hard. Particularly in the later levels. But simple enough so as not to get bogged down in the technicalities and just enjoy killing the titans. Love the retro graphics and music too. Played a bit of this for Steam badges in Christmas 2011. Only just now getting back to complete the main campaign. Yet another Humble Indie Bundle win. Might keep going with the titles from them...

more steam objectives. revenge of the titans is pretty awesome. hopefully will get back to that. and definitely looking forward to a full co-op run on serious sam 3