On a part of the fortifications of Vauban Citadel in Lille (north of France), during a restoration of counterguard, an experiment for the biodiversity has been conducted. It must allow some plants and animals species (invertebrates, bats) living on the site, and allow the public to observe them. Here, the Architect of the buildings of France authorized an experimental construction : a few meters of gabions filled with small hard limestone stones and earth, and (over 100 meter in length) he authorized (the long of a small pathway) a little wall of special blocks, assembled with clay and filled with compact poor soil (made from clay and lime).Interest : natural spores and seeds of wild plants, adapted to this environment comes with wind, insects, birds, etc. There is self-selection of species. Many insects and invertebrates will be able to enjoy the microhabitats that occur naturally in the wall, that should not require maintenance. A self-adaptive living communities in this microclimate is expected (hot and dry conditions were very windy).