How will existing cities evolve? Brussels, a small capital which has become a European headquarter, is a perfect case study of a city that has had to transform under the pressure of new needs. In a century, it will have changed again to adapt to living conditions without fossil fuels. Slowly, little by little, this city, becoming too urbanised and mineral, will reduce its tarmacked and cobbled surfaces. It will remember its streams, its valleys, and its ponds and rediscover the quality of life that it had lost. Its architectural art will be maintained next to shapes brought by new vegetal structures, grafted onto existing buildings. The big city-centre car park will be transformed into a landing pad for ornithoplane with beating wings.