In this paper, we propose a methodology developed in the framework of the VISPO project for engineering a three-layer ontology, based on the conceptualization, integration, synthesis and categorization of XML data descriptions provided by a number of sources in a virtual district, where different enterprises cooperate for business purposes. Ontologies are proposed as an unifying framework for different viewpoints by providing a shared understanding in a subject domain. Our methodology generates an ontology organized into concepts and concept relationships at different levels of detail, to provide multiple, unified views of the datasources containing heterogeneous information about the domain of interest.