Radzyń Chełmiński is situated in the northern part of the Chełmno Land. In the Middle Ages, an important route from Pomesania led through that area. In that strategic place, most probably on the basis of an early medieval settlement, the Teutonic Knights led by Herman Balk erected the first fortress in 1234. In 1243, it was one of a few which was not conquered by Prussian rebels. In mid-13th century, it became a komtur’s seat. As the archaeological research has shown, the masonry castle was erected in a new place. On the basis of the research, as well as the architectural analysis of the preserved walls and size of bricks, it is possible to reconstruct the building process of the fortress. In the first phase, in the following stages, which had basically been pre-planned, the convent building was erected. In the first stage, a curtain wall of the convent building was built in narrow-space foundation trenches and, at the same time, window openings, vault prop and toothing for future partition walls of the flanks were prepared. After erecting a curtain wall, the level of the area around the convent house was raised by about 2.5 meters, creating an artificial hill. In the following stage, the main (southern) flank was erected. On the level of the cellars, short rudiments of walls of the neighbouring flanks were built, leading out from its northern wall. Other flanks of the castle were built later. On the basis of the consistency of architecture and the measurement analysis of the bricks, it should be stated that the application in various parts of the building of the Wendish (cellars) and Gothic (ground parts, some partition walls in the cellars) brickwork does not mean that the phases of construction of those elements were distant in time. The sizes of the bricks used in the entire building are consistent irrespective of the brickwork. The use of the Wendish brickwork in the cellars can be explained by the fact that it was used for facing the stone wall, unlike the Gothic brickwork, which was in full brick walls. The buildings erected in the second stage of construction of the castle were made of brick of a visibly larger size. At that time, the bergfried was probably erected, as well as the outer ward with a dansker and an outer bailey. The use of various types of brickwork in the curtain wall of the latter should be explained by work of various teams of bricklayers. In connection with the construction of the elements of the castle listed above, further earth works were performed, that is, the level of the outer ward was elevated and made even, the plateau of the outer bailey was widened from the south and an embankment was constructed in front of its western curtain. The last medieval building works comprised the erection of buildings on the outer ward of the convent house. The chronology of construction of the castle was examined by researches in different ways. On the basis of forms of gables of the main flank, it can be determined that the construction of the convent house was completed in the 40s of the 14th century. Thus, it was probably undertaken before the congress of the dignitaries of the order, which was held in Radzyń in 1329. Perhaps, during the congress, the works on the rooms in the main flank of the castle (the chapel and the refectory) were about to be finished. On the other hand, it should be estimated that the outer ward and the bailey were erected in mid- and second half of the 14th century.