Harmony Square is a short, free-to-play browser game developed in collaboration with the University of Cambridge, DROG, the U.S. Department of State's Global Engagement Center, and the Cybersecurity and Infrastructure Security Agency. The player assumes the role of Chief Disinformation Officer in an idyllic neighbourhood and is tasked with sowing discord: creating misleading content, exploiting societal tensions, trolling, and deploying conspiracy theories to polarise the residents. The premise is deliberately transgressive — the player succeeds by spreading disinformation — but the pedagogical purpose is precisely the opposite. The game is grounded in inoculation theory, a psychological framework that posits that exposure to weakened forms of manipulative argumentation builds cognitive resistance, much as a vaccine builds immunity. A randomised controlled study published in the Harvard Kennedy School Misinformation Review found that playing the game improves players' ability to spot manipulation techniques, increases their confidence in doing so, and reduces their stated willingness to share manipulative content. Harmony Square belongs to a family of inoculation games that includes Bad News and Go Viral!, each targeting a different vector of misinformation. The Academy hosts it in the World School because its central exercise is the modelling of an information ecosystem: the player learns the grammar of manipulation by constructing it, and leaves the game better equipped to read it in the wild.