The soil structure interaction is usually omitted in the seismic analyses of ancient historical buildings, considering the fixed base condition. However, usually, these buildings present a poor foundation, and they are over soft strata, making the assumption of the fixed base condition unrealistic. This paper aims to assess the seismic behaviour of the well-known Giralda tower (Seville, Spain) considering the soil-structure interaction (SSI). It was built with unreinforced brick masonry walls in the XII century. A 3D nonlinear numerical model based on the finite element method has been developed, considering the foundation and the soil. Nonlinear static and dynamic analyses have been carried out in the OpenSees framework. The SSI has been simulated through the direct method. The interaction with the Cathedral has been borne in mind. The results have shown that, if the soil structure interaction is considered, the peak strength is similar to the fixed base model, but the displacement is approximately doubled. This leads to an increase of the damage expected.