Documentation, inspection and structural analysis are fundamental techniques to understand the behavior of historical constructions, determine their vulnerability and to propose interventions. The interventions on heritage buildings and their efficiency are highly related with the knowledge of the structure. Within this framework, an assessment focused on the Chapel of the Fortress and Convent of Ínsua de São Isidro, placed on an inhabited island in the northwest of Portugal, is presented. The study of this monument represents a challenging task, due to its severe deterioration, mainly caused by weathering and inadequate interventions, and its location (limited access). Nonetheless, the analysis was carried out using three approaches. In the first approach, a detailed historical review and general visual inspection of the entire complex were done. Next, the visual inspection and non-destructive tests in the chapel were carried out, aiming at obtaining the properties of the structure. The non-destructive tests included sonic tests, impact-echo tests, GPR tests, photogrammetry and dynamic identification tests. In the third approach, a numerical model of the chapel was prepared and calibrated. Finally, the structural assessment for the vertical and seismic loading was performed. The results showed that the chapel presents an appropriated load capacity. However, interventions to safeguard the integrity of the chapel must be considered. Furthermore, further studies are recommended, aiming at validating the conclusions obtained from the numerical modelling.