Chełmno began to develop dynamically in the late 13th century. The town’s prosperity was built on transit trade, based on contacts with Poland, Ruthenia, Hungary and Flanders. It was a member town of the Hanseatic League up until the end of the 14th century, though it was gradually being displaced by Toruń. The Teutonic Order even undertook to establish a university in Chełmno, resulting in an act of foundation being granted by Pope Urban VI in 1386. After the Teutonic Knights were defeated at Grunwald (Tannenberg), Chełmno, along with other Prussian towns, surrendered and paid homage to the king of Poland, before going on to play a significant role in the Prussian Confederation, formed in 1440. After the Thirteen Years’ War and occupation by mercenary forces, Chełmno once again came under the Polish king’s rule in 1479. In 1505 the town was granted to the bishops of Chełmno by Alexander Jagiellon.

Chełmno provides an excellent illustration of urban planning, with its elliptical layout and central market square measuring 157 × 113 m. The main feature of the market square is the brick-built town hall, raised in the 14th century and comprehensively remodelled in the 16th century, when it was surmounted by a slender tower, producing one of the more interesting examples of Mannerist architecture. The town’s streets intersect one another at right angles creating a grid of square and rectangular spaces filled with groups of buildings which still conform to their original medieval building plots, laid out using a unit of measure known as the Chełmno rod.

Virtually the entire circuit of the town’s medieval perimeter walls survive intact, together with 23 towers, but only a single gate - the Grudziądz Gate. The walls follow the line of the escarpment’s high ridge, exploiting its excellent defensive properties. Easier accessible areas, i.e. from the east and west, were protected by water-filled moats - these were replaced in the 19th century by parklands referred to as the Planty, which currently separate the old town from the rest of the city forming a green belt around the historic complex.

The Church of the Assumption of the Blessed Virgin Mary, built in 1290-1333, is a monumental structure; noteworthy features of its interior include carved bosses and portals, as well as statues of saints made in artificial stone. These harmonise with the church’s Gothic polychrome decoration. In 1560 the castellan of Chełmno, Jan Niemojewski, founded the Chapel of Our Lady of Sorrows. The church was partially renovated in 1659-1660; it was also at this time that the Corpus Christi Chapel received its Baroque décor. The Franciscan Church of St James and the Dominican Church of SS Peter and Paul were raised in the 13th century and rebuilt in the 14th century. Architecturally more modest, though with a delightfully lavish interior, is the Church of St John the Baptist, formerly part of a Cistercian nunnery (13th century), and later a Benedictine one.

The superbly preserved old town, complete with its defensive walls and major ecclesiastical and secular buildings, is the pride of Chełmno; however, its greatest significance is in having provided a model of urban laws and settlement which was successfully adopted in other regions.