## 1 Introduction  
In the last decades, the necessity to document and store information related to cultural heritage (CH) and historical built heritage (HBH) has led researchers and experts to focus on the techniques and methods to more effectively represent historical buildings, geometric features and knowledge, spatially. Most recent methods and technologies in this respect adapt building information models (BIM), born to assist in building design and construction for the representation and management of heritage information, known as historical BIM (HBIM) [1, 2]. However, the geometry usually stored in BIMs is parametricallymodelled, while a large amount of the explicit geometric information (i.e., stored by means of the values of coordinates of single points) can be available for historical buildings from advanced survey techniques. The automatic interpretation of such information would be an advantage in the conversion of point clouds to BIM. In addition, the use of an effective ontology to define proper semantics for historical building objects already in this early modelling phase would greatly support the following management and interpretation of the building knowledge.  
The process and the activities needed to preserve, valorise, restore, and document CH and HBH within spatial and temporal dimensions involve many communities, use cases and actors dealing with various data acquisition and processing phases. For this reason, a common language to semantically represent HBH is required.  
An ontology or a complete conceptualisation that is able to semantically formalise historical buildings and HBH does not exist. In the last decades, many standards, vocabularies, and some ontologies for representing the built and architecture domain have been developed and adopted internationally (Section 2). However, none of these can spatially define entirely architectural elements of historical buildings and ancient structures, and their relations with the urban context and the historical centre in which they are located.  
Considering the most widespread and accepted definition and understanding of the term ontology [3; 4; 5], and the significant role of ontologies with respect to knowledge engineering [6] and artificial intelligence, an ontological structure would be an effective solution for the formal conceptualisation of the HBH domain [7]. Hence, in this paper we propose an ontological structure useful from the early modelling phase of point cloud interpretation and segmentation to the later phase of management of the stored information. Such a workflow allows the connection between semantic and geometrical/topological information to generate structured models of built heritage.  
Section 2 presents the state of the art on the relevant topics involved. Section 3 is dedicated to the methodology workflow, with a specific choice of archetypal case studies for HBH types included in castles and towers types (Section 3.1). Section 3.2 is aimed at analysing what the concepts are and identifying characters of such fortified structures by incorporating definitions and characteristics, and classifications and hierarchies of components according to standards and other published sources, as well as identifying ontology classes (objects) definitions (Section 3.3). Section 4 is devoted to the ontology development, and subsequently, the validation through semantic segmentation is presented starting from point clouds models (Section 5); the results are discussed, highlighting the connections with the ontological scheme.