Code Critters is a serious game designed to make software testing an engaging and integral part of learning to program. Set in a colorful tower defense world, players use magical tools to distinguish between critters executing correct and buggy code. By placing enchanted portals, players implicitly create test cases and define expected behaviors, turning testing into a natural and fun activity. Designed for younger audiences and built on a block-based programming language, Code Critters introduces key testing concepts such as test inputs and oracles. Recent developments extend the game to include loop-based programming, allowing deeper exploration of fundamental coding constructs. Empirical studies with children and teenagers show that Code Critters fosters active engagement and learning—both in classrooms and beyond.