The city of Belfort was planned to control the pass between the Jura and the Vosges.It’s pentagonal plan, was designed in 1687 by Vauban, completed and reorganised by the General Haxo from 1817 and on. The Fort Hatry where the sports and events arena of Belfort is located was one of the exterior bastions of this defence device.The fortifications have a geometry not derived from the urban orthogonal grid but with multiple directions designed to control movement, conceived to obtain clear sight lines, designed for the art of war, with an aesthetic of edges and folded walls.Since it’s dismissal as a defence device the Fort Hatry has become an informal park used for open air activities, leisure and events, giving place for ephemeral structures such as luna park’s and circuses. It is today a transitional space, an infrastructural knot where roads and railways cross at different levels and at different speeds.Conceived as a powerful landmark, the new building establishes a strong relationship with the majestic skyline of the nearby Vosges mountains and the Belfort Castel.The facility assumes its role as a public building and urban catalyst, a place of centrality, gatherings, meetings, sport and cultural events.The design of the new building creates a strong contrast to the existing stone walls. Recalling the fluidity and the dynamic of sports activities, the façade of the sports arena makes a flexible and generous movement through the landscape. Convex and concave folds react to the existing straight lines.The envelope of the building, realised in translucid glass with a smooth and crystalline quality picks up the subtle colouring of the natural light, the sky and the landscape.At night the building is transformed into a bright lighthouse, fully expressing the events taking place inside.The building is firmly anchored in the hillside, the glass skin is cut to fit perfectly in the artificial topography, in the same time the glass ribbon seems detached, almost floating over the pre-existing fortification.The new structure seams to have always been there, responding to the logic of the site and in the same time it appears as something as light and ephemeral as a circus tent.The building expresses two aspects of sport activities, on one hand the discipline and the rigour, on the other the playful sensuality through the contrast between the orthogonal geometry of the playing field, and the free lines of the envelope.The glass ribbon enlaces the programmatic activities, creating compressions and expansions between the halls and the skin.The main sport hall is the heart of the complex, it is served by a public circulation directly open towards the playing field. By positioning this space around the hall, it forms an open liaison; an intermediary layer between the hall and the city.The stands are concentrated on the two long sides of the playing field, giving a more dramatic section and intensity to the space.In this way the two short sides stays open towards the entrance hall and the public circulation, making place for standing viewers with a different, fluid and informal perception of the game.The proximity between the players and the public aims to dissolve the traditional barriers between those who play and those who watch.