In the Dutch delta has been acted on water safety for centuries. As a result of this ongoing water works, parts of the existing dike system originate from ancient times. In this paper, the safety assessment of five structures is considered and compared with the Dutch flood safety standard. These structures are part of an historic fortress build around 1700, and must comply with modern safety standards. In order to complete the safety assessment, the various process steps are explained to get to an informed decision. The constraints and opportunities for field research and analysis are explained. These constraints are the result of the diverse functions of the structure besides water safety and interests of the various stakeholders. The feasibility of the user expect ations in relation to the structural strength had to be determined, based on factual geotechnical data and observed behaviour of the structures.