Really cool game! I love the design and how clean and sharp everything looks, and the idea is pretty cool as well. If you're looking to improve this, maybe add more things that will want people to continue to play your game, for example: a reason to get score, (like a place to spend it), levels, more types of cubes that appear as time goes on, maybe even rockets that you have to shoot before they hit you. Fantastic idea, needs to be made into a game :P
Beautiful. *sheds tear*
Anyway, the only reason I beat the game is because I left the browser open while I was doing something else in another browser, so maybe fix it so that setInterval only executes when you are looking at the game.
It's really easy to win. The same strategy works over and over again:
1. Put an X in the bottom left square.
2. Put an X in the top right square.
3. Put an X in the bottom right square.
4. Put an X in the middle right square.
YOU WIN!
Works every time.
Found a bug. If you click space right after the level ends, it goes to the next level, then the Win screen will come up and ask you if you want to go to the next level, and then you can just click it and you advance 2 levels at once...
 "Woodclicker"){kind=icon}
 "Apocashop"){kind=icon}
 "Shoot the Box"){kind=icon}
 "Zumbi Blocks"){kind=icon}
 "This is the Only Level 3"){kind=icon}
 "Give Up"){kind=icon}
 "Roll A Ball"){kind=icon}
 "Ragdoll olympics"){kind=icon}
 "POTATO POTATO POTATO (etc)"){kind=icon}
 "Thermo"){kind=icon}
 "Mara"){kind=icon}
 "Knightmare Tower"){kind=icon}
 "DropStep"){kind=icon}
 "Tic Tac Toe(HTML)"){kind=icon}
 "Idle Empire 2"){kind=icon}
 "Pixelo"){kind=icon}
 "Monster Bark"){kind=icon}
 "AdVenture Capitalist"){kind=icon}
Thanks :)