This paper focuses on the seismic behaviour of the Casamari Gothic church located close to Frosinone in Italy. Linear static and dynamic finite element analyses are developed using a three-dimensional model of the whole church employing the commercial code ABAQUS. The three-dimensional model was realized using shell elements to model the midplane masonry walls. Fundamental frequencies of the cathedral in the main horizontal directions are derived by means of the finite element analysis. Non-linear static analysis on a transversal section with horizontal forces adopted to represent the seismic action, known as pushover analysis, has been carried out to estimate the seismic response of the structure. The analyses have been carried out under conditions of constant gravity loads and monotonically increasing horizontal loads. In this case, the finite element model will be a two-dimensional plane stress model. In particular, masonry has been schematised as an anisotropic medium, whose yield criterion is derived using homogenisation, starting form a block structure with periodic texture. The adopted macroscopic model is shown to retain memory of the mechanical characteristics of the joints and of the shape of the blocks.