http://jsfiddle.net/plamoni/aMFwr/embedded/result/

Again, I used jQuery because I’m a rebel.

Seems to work best in Safari, but appears to work fine in other modern browsers. There are a few minor glitches in timing, but aside from that it’s very playable. What I mean to say is that it’s reached the point all casually written computer games reach: Where it’s complete enough that you spend more time playing than coding and lose interest in the latter in favor of the former.

This challenge was pretty free-form. I’m looking forward to what others create!