Recently, in the Cuban capital of Havana, a number of fortresses were given away by the Army for refurbishment and conversion to public functions. In the last two decades, there have been examples of refurbishment and conversions of fortifications in museums, some of them with dire consequences for the collections and the structures. In this type of structure every architectonical element has historic value and very few interventions are allowed. However, the accommodation of modern functions and the preservation of the structure heavily rely upon the balance between various environmental parameters of the interior spaces. This study explores the challenges and opportunities of an environmentally driven conservation approach to the refurbishment of one of Havana’s most iconic XVIII century’s defensive structures, Santo Domingo of Atares Castle. The paper illustrates the development of an architectural proposal for the conversion of the castle into a museum and associated energy and environmental strategies. The museum function is particularly demanding due to the rigorous standards for the preservation of the collections, generally accomplished by mechanical equipment. Recently, however, there has been a major concern in the conservation scene about energy consumption and sustainable practices in this type of buildings. Therefore, the work identifyies the characteristics of the existing climatic and environmental conditions affecting the interior spaces of the thermally heavyweight fortress. This has led to an analysis of the requirements of the collection in a hot-humid climate in conjunction with the parameters for human comfort, in order to identify to what extent the original conditions can be passively modified to adapt the building to the conversion. Finally, with the help of computational dynamic simulations, passive strategies and zoning options were tested for the achievement of suitable interior environmental conditions and subsequent energy savings.