In the framework of conservation and enhancement of cultural heritage, accessibility plays a crucial role. Architectural heritage peculiarities call for specific approaches: the same care must be adopted to improve accessibility to historic architecture, designing actions case by case. Starting from these considerations, the contribution is aimed to discuss the accessibility of fortified architecture, a not easily accessible heritage, to make it accessible both in a physical meaning as well as in cognitive, cultural and social. Particularly, the authors propose some reflections about alternative ways of usability for inaccessible architectures due to their typological configuration and as a consequence of previous conservation design. The case study is the medieval castle of San Michele in Cagliari (Sardinia), which was transformed several times during its life and restored at the end of the 20th century. The contribution analyses how a multidisciplinary approach is necessary for the formulation of effective accessibility solutions, especially in complex cases such as fortified architecture. This multidisciplinary approach must be accompanied by the widest possible consideration of the causes limiting accessibility, whether they are physical or related to motor, perceptive or cognitive disabilities.