The "Level Failed" screen should appear after the monster stops moving and not after a certain amount of time after the last cut. At some levels you can clearly see that the monster would touch the spikes after a while. Beside of this: Very nicely done game. Congrats! 4/5
@SilenGames: There is definitely a save game problem. I'm on Ubuntu 11.04 and use Firefox 5. It happened two times, that the game crashed. After a reload, my progress is reset to level one, no skills, no money, no upgrades. Just the achievements remain on the correct amount.