This paper presents a project designed and built for the Seventh World Urban Forum (WUF7) in Medellín, Colombia, commissioned by the Department of Habitat of the United Nations (UN-Habitat). The project is a “free-form” masonry shell, built using the technique of thin-tile vaulting (also known as Catalan or Guastavino vaulting). UN-Habitat intended this shell to be a demonstration for the technique’s efficiency, versatility, durability, sustainability and economy as an example to be exported to developing countries. The shell’s shape was designed using the form-finding software RhinoVAULT, which allows the design of compression-only surface structures. Special care was taken to integrate the vault in its surroundings, a park in an urban area of Medellín. It wanted to be visible, but at the same time also a natural part of its environs. The shell emerges from the earth to shape a new topography featuring a green roof that continues the park’s landscape, while showing a surprising brick-patterned interior, creating a warm, cozy space as a contrast to its exterior. The design process, structural analysis and production of an efficient falsework and the different processes related to the construction are also detailed in this paper.