As gaming and gamification play an increasingly important role in recruitment processes and radicalisation, there is an urgent need for evidence-based research in this field. One aspect is the use of games and gamification in prevention work. The article presents a project in which an online-game against extremism was developed and focuses on its evaluation using a pre-post design and a combination of quantitative and qualitative methods. The aim of the game is to educate young people about radicalisation processes in order to increase their resilience. The pre-post comparison showed that young people changed their attitude towards extremist narratives after playing the game: they agreed significantly less with statements that referred to extremist narratives, e.g. legitimising violence or spreading conspiracy theories. When they played the game in the course of a workshop, they also showed lower approval rates for authoritarian attitudes afterwards. The self-assessment of their learnings was consistently high, whereby even greater effects could be observed for those who had played the game in the course of a workshop. Despite some limitations in data collection due to the COVID-19 pandemic, the evaluation provides interesting insights into the impact of the game on the prevention of radicalisation.