After not having a good idea for quite some time, I decided to create an extra challenge for me this time. So, the whole game is based on a single image, which can be found at http://www.weirduniverse.net/blog/comments/humane_bullfighting/

Use the arrow keys to move around. Moving over critters kills them (You monster!). Ramming attack trigger automatically.
Your goal? Escape the arena.
How to? Kill. Earn experience. Buy upgrades. Break the fence.