Given that this game is £10.99 for one copy, I think this could well be a pricing error, so be quick and grab a 3 pack of Particula for only 53p.
"Try, try one more time. Life is full obstacles, overcome them! Destroy negativity, clear your mind! Particula brings you a new perspective in gaming, helping you to concentrate, and to clear your mind. Live, live like it was your last day.
- 3 Players Co-op tower defense. Play it with your friends!
- Freeze, burn and purify
- Random enemies, Waves and Bosses
- Smooth soundtrack
- Steam Trading Cards"
Full credit to Jaystan at HUKD for the find.