Learning to program has become common in schools, higher education and individual learning. Although testing is an important aspect of programming, it is often neglected in education due to a perceived lack of time and knowledge, or simply because testing is considered less important or fun. To make testing more engaging, we therefore introduce Code Critters, a Tower Defense game based on testing concepts: The aim of the game is to place magic mines along the route taken by small “critters” from their home to a tower, such that the mines distinguish between critters executing correct code from those executing buggy code. Code is shown and edited using a block-based language to make the game accessible for younger learners. The mines encode test inputs as well as test oracles, thus making testing an integral and fun component of the game.