Castle Defense is an action-packed math game. It was written from scratch for the Liberated Pixel Cup contest, purely in HTML5, Javascript, and CSS. It uses layered canvases, sprite-based graphics, local storage, and HTML5 Audio to create a fully immersive experience. It runs well in all of today's modern browsers, best in Fullscreen mode in Mozilla Firefox. To play, type the answers to activate your archers. Kill the skeletons before they reach safety. Use the number keys on your keyboard and the brain in your head. Good luck. The current version can be played here: http://castle.bipsland.com