In the presented work, a comparative numerical study of 17th century cathedral-turned-into-mosque preserved in Algiers, the capital of Algeria is described. Ketchaoua Mosque is a masonry structure with a rectangular plan representing features of cross-shaped cathedrals and is located at the foot of the Casbah, a UNESCO World Heritage Site. The structure has recently been monitored and analyzed because of an emerging damage on minarets and some part of vaults due to some important seismic events at the last decade. Dynamic identification tests have been carried out for determining the dynamic properties of the structure. In order to evaluate the existing seismic capacity of the mosque and select the appropriate retrofitting technique, incremental nonlinear static analysis of the structure is performed for gravity and seismically originated forces by the means of finite element method using DIANA software. Several numerical simulations are performed starting with linear elastic model comprehending the global structure. Finally, the results of system identification are compared with the results of the numerical models and future research proposals are outlined.