Masonry arch bridges represent a significant portion of the existing infrastructural heritage in Italy, often serving as critical elements in local and regional transportation networks. Given their historical value and widespread use, ensuring their structural safety under both static and seismic conditions is a matter of growing importance. This paper presents a code-compliant assessment of a historical masonry arch bridge, with a focus on how different levels of knowledge affect the evaluation of its safety. Starting from an initial analysis based on conservative assumptions and standard material properties from design codes, the study progressively incorporates more refined information obtained through in-situ mechanical testing and detailed inspections. Numerical models corresponding to each knowledge level are developed in accordance with current national and international standards. The influence of the available knowledge on the estimated safety margins is evaluated under both gravity and seismic loading. The results highlight how improved knowledge can lead to more accurate and potentially less conservative safety assessments. This work underscores the importance of knowledge-driven approaches in the structural evaluation of existing masonry bridges, offering practical guidance for preservation and risk mitigation strategies.