The Cathedral in Chełmża is one of the oldest and most significant brick churches in northern Poland. The Chełmża bishopric was established in 1243. Several years later, the construction of the church walls began. It is a three-aisled hall with a straight-ended presbytery and transept. The western part is closed by a two-tower facade with a rosette. Stellar vaults dominate the interiors. The purpose of this article is to present the results of architectural, metrological, and structural research that has allowed for a detailed tracing of the building's history. To achieve this, the entire building was first scanned. Subsequently, floor plans, sections, and elevations were drawn, which served initially for the analysis of the brick structure to determine the size of the bricks and the bonded and added corners. Furthermore, a metrological analysis was conducted, based on which it was determined that the entire church was surveyed from the very beginning, but only the presbytery, transept, and north wall of the nave were built by the end of the 13th century. The western towers and the nave were constructed in the first half of the 14th century. To verify whether the proposed reconstruction of the building's state at the turn of the 13th and 14th centuries could have actually existed, additional static analyses were conducted.