Zagreb leverages its status as one of Europe's greenest capitals to expand climate ambition into previously underutilised spaces — private and semi-public courtyards. While the city boasts abundant green infrastructure, many inner courtyards remain untouched by climate action. ActGREEN activates these spaces as carbon sinks, enhances liveability in dense urban areas to counter heat islands and suburbanisation, fighting urban sprawls and fostering inclusive citizen participation. The approach combines ecological restoration with citizen-driven innovation, positioning courtyard greening as both climate mitigation and urban quality-of-life improvement.