Naval wargames play an important role for both training of naval cadets and trying out tactics for naval warfare. Constructing an ontology can be regarded as a way to formalize the mission space component of a simulation conceptual model. An ontology has been developed to capture the entities involved in a naval wargame and the relationships among them. The ontology is defined in OWL (Web Ontology Language), using the Protege ontology editor and knowledge-base framework. This paper presents the ontology development process, discusses the decisions made throughout the creation of the ontology, and identifies the lessons learned.