The cathedral complex was an independent urban unit with an elongated rectangular layout (163 × 80 m). The Cathedral of the Assumption of the Virgin Mary and St Andrew the Apostle, along with its surrounding buildings (the Chapter House, Curator’s House, Chancery Office, and the New Vicarage), occupies the site of the original episcopal stronghold. Construction work on the cathedral began in c. 1330, and it was consecrated in 1342. The cathedral’s design combines features noted in Teutonic (chancel), Cistercian (nave) and Rhineland (gable) architecture. It is a three-aisled building measuring c. 97 m long, its width ranging from 12 m in the chancel to 22 m in the nave. The interior is roofed with star vaulting. Most of the lavish decoration dates from the period following the first Swedish invasion, i.e. after 1626. The cathedral boasts a fine organ, built in the Gdańsk atelier of Daniel Nitrowski in the years 1683-1686. The Bishop’s Palace, originally a Gothic-Baroque building of the 14th-18th centuries, stands in the south east sector of the fortress complex. It burned down in 1945 and was subsequently reconstructed between 1965 and 1970 as a museum.

Fortifications comprise straight stretches of brick walls reinforced with bastions, and afforded protection by gullies and a moat. These defences were raised in the 14th century and periodically modernised up until the 1620s. The fortress was provided with two entrance gates and a doorway. In the late medieval period the fortified cathedral complex was besieged and captured by Polish, Teutonic and Czech armies. Particularly severe damage was inflicted by the Swedes in 1626-1630 and 1655-1660. In 1843 and 1856 parts of the fortifications were dismantled. One corner of the curtain wall was protected by a large, polygonal artillery tower surmounted by a belfry. North-west corner tower is referred to as Copernicus’ Tower, although the eminent astronomer (whose tombstone can be seen inside the cathedral, his body being interred in the crypt) never actually used it for his celestial observations, which were made instead from a canonical residence beyond the defensive walls.

Seven extramural canonical residences and a new Bishop’s Palace are located outside the confines of the cathedral complex. The residences were home to those who held the office of Canon of Frombork, among them Stanisław Hozjusz and Ignacy Krasicki. Those buildings which are still extant date from the 18th-20th centuries. Some of them have Gothic cellars, one of which was raised on the foundations of Nicolaus Copernicus’ house. Copernicus was Canon of the Warmia Chapter and lived in Frombork from 1510 until his death. It was here that he wrote his famous work De revolutionibus orbium coelestium, propounding his heliocentric theory of the universe which revolutionized science and our perception of the world. The Frombork hill complex is a unique cathedral fortress, harmoniously combining various types of building: fortified, ecclesiastical, residential and administrative. Copernicus and his treatise elevate Frombork to a position of worldwide significance. It was here that the following ground-breaking words were penned: “The first and the highest of all is the sphere of the fixed stars, which contains itself […]. It is followed by the first of the planets, Saturn […]. After Saturn, Jupiter […]. Then Mars […]. The annual revolution takes the series’ fourth place, which contains the Earth, as I said, together with the Lunar sphere […]. In the fifth place Venus […]. Lastly, the sixth place is held by Mercury […]. At rest, however, in the middle of everything is the Sun. For in this most beautiful temple, who would place this lamp in another or better position than that from which it can light up the whole thing at the same time?”.