<p>This study introduces a novel methodology that combines advanced 3D modeling and custom computational tools to investigate solar illumination in ancient monuments, using the Kastas Monument in Amphipolis as a case study. By enabling precise analysis of light-shadow interactions, dynamic sensitivity analysis, and error quantification, the approach generates new insights into the monument’s architectural and symbolic design. Key findings suggest that during its second construction phase, several architectural elements of the monument were deliberately aligned with specific solar phenomena, peaking during the winter solstice but extending throughout the year. These alignments are conceptually linked to themes of renewal and life, particularly the cult of Cybele, as reflected in the monument’s architectural features and iconography. The study also examines the theoretical possibility of a missing architectural element, such as a statue before the burial chamber, that may have interacted symbolically with solar illumination. Beyond the Kastas Monument, this methodology provides a replicable framework for integrating computational tools with 3D modeling to explore solar alignments and architectural design in ancient structures. By combining precision with flexibility, it facilitates sensitivity analyses across diverse datasets, making it applicable to a wide range of cultural and archaeological contexts. This study not only contributes to understanding the cultural astronomy of Macedonian architecture—a previously underexplored area—but also offers a versatile approach to advancing research in cultural astronomy, architectural studies, and the broader field of ancient monument analysis.</p>