In this paper, we presented the Towntology project whose objective is to design an ontology in the urban field. This project is divided into several stages. Concerning the prime main objective of the Towntology project, i.e. to build an ontology for urban planning by integrating all urban objects with their descriptions and relations, we began to deal with some fields, such as street planning and mobility. We also developed a software tool making it possible to urban planners to work on a semantic network. Thus we built a visual browser and an ontology editor. An extension of XML is designed and used for the description of the ontology.