The Trakošćan Castle in Croatia stands as a cultural heritage property, with a construction period spanning from the 13th to 18th century, reflecting centuries of history and architectural evolution. Situated in a seismic-prone region, the castle faces constant vulnerability to earthquakes. While old masonry buildings are typically capable of bearing vertical loads, they often lack the structural integrity to withstand the lateral forces, as demonstrated by the significant damage due to the central Croatia earthquake, emphasizing the need for seismic assessment and conservation measures. This study aims to evaluate the seismic performance of Trakošćan Castle using the Finite Element Method in conjunction with in-situ measurements. In the first step, a comprehensive 3D Finite Element model is created based on existing architectural drawings in DIANA v10.7. The model is then calibrated by aligning the analytical frequencies with the fundamental vibration frequencies obtained from in-situ tests conducted in 2016 and 2021. Following calibration, the structural response to seismic events is evaluated using linear response spectrum analysis. The results are scrutinized, with particular attention given to displacements at identified critical nodes, tensile stresses, and shear stresses. The results provide an understanding regarding the structural performance, indicating the structural members that are susceptible to damages, where material stresses are either exceeded or are close to the allowable design capacity.