Revenge of the Titans is the best tower defense game I’ve played so far. RotT has skill trees that span about 5 different “worlds”. The game plays differently depending on what skill tree you pick. There is a Campaign, Endless, and Survival mode. One thing I did not pay attention to is that Survival mode uses your research tree from the last level in that world. This can definitely impact your Survival Mode times if you were holding off for something critical like barriers until just after the next world. Anyway, I managed to get the top time on survival and held it for a few weeks. Right now the record is around 3 hours (scratch that, 6 now!), which is super hard to do. No, you don’t have to play a level for 6 hours, you can hold down Tab to fast forward the level. So far I’ve done at least 3 playthroughs. Once just after the Humble Bundle, another a few months later just before tank droids were added, then again just after.
My one gripe is that the research trees are hidden except for your next “level” up. I found that very annoying and ended up looking up the wiki. Some people claim that is a spoiler, but I disagree. There’s an algorithm that adjusts the difficulty depending on how much cash you are saving up, so you are always kept on your toes. The basic summary of the algorithm is that the more money you end up making and saving, the harder enemies are. This ends up keeping you on the edge of destruction quite a bit. I restarted levels several times after my approach failed, but never picked the restart with easier difficulty option.
This game has a high re-playability factor and is also highly addictive. After a few nights of multi-hour gaming sessions, I found myself dreaming of placing towers in just the right position to maximize my cash. The survival mode is brutal, but satisfying when you see your name rise up on the leader board. I highly recommend RotT for anyone that likes tower defense games. Sure, there are dozens of free variants you can play in a web browser for free, but I don’t think any of them have he same depth as Revenge of the Titans by Puppy Games. Check out the official site, or the Steam store link. Keep in mind that if you purchase through Steam, you do not get free access to the Linux version, just Mac and Windows.