The phenomenon is visible: nature today is at a critical point, and cities are vulnerable. One of the leading causes is the ever-increasing process of soil sealing that has involved cities from the twentieth century onwards. Climate change and soil sealing have weakened the cities that can no longer control natural events. Today, the increasingly prosperous and ever-growing cities must face an issue as crucial as it is urgent: the relationship between ecology and nature, between built and nature. If, on the one hand, water represents a threat to the city, on the other, one of the main resources to deal with climate change effect within urban voids. Starting from the assumption that ever-growing urbanization negatively affects all ecosystem services, and that water is a fundamental resource for ecological restoration and mitigation of the effects of climate change, four European case studies of urban regeneration through the rethinking of public spaces through nature-based solutions are highlighted. The case studies have a recognized architectural quality, belong to the European cultural context of open spaces, and are in metropolitan areas; they are selected following defined variables: reduce heat island effect, improve air/soil/water quality, increase biodiversity, and reuse of urban water.