The Archbishopric of Cologne secured its border with the County of Jülich-Berg in the 14th/15th century with a series of forts. One of these, within sight of the Eifel mountains, was the fort in Lechenich. What these forts have in common is that they were largely built of bricks.
In 1306, the Archbishop of Cologne, Heinrich II von Virneburg (1305-1332), began building a new castle within the city with the permission of King Albrecht. The residential tower was built between 1306 and 1317. During the reigns of Archbishops Walram of Jülich (1332-1349) and his successor Wilhelm of Gennep (1349-1362), they had the castle extended like a fort. Two wide moats secured the complex. The outer moat surrounded the entire site, the inner moat enclosed the main castle. Further protection was provided by the town, which was fortified with walls and ditches.
Building with field-fired bricks was a new technique at the time, which had been reintroduced to Germany from Italy (after Roman times) by the Staufers in the 12th century. The Fort Lechenich was probably the first large building to be built using this technique. Nevertheless, trachyte from Drachenfels was used on the corners and the window frames. After the persecution of the Jews in 1349, gravestones from the abandoned Jewish cemetery in Cologne were reused as spolia in the outer and main castle. The tombstone of Mar Jacob at the gate of the outer castle is well preserved.
Outside, the five-storey keep is 15 metres long and 13 metres wide. The walls have a thickness of 2.50 metres at the bottom, decreasing by 20 cm from floor to floor. The high castle (palace) occupies the entire east side of the complex, with a length of 33 metres and a width of 12 metres. This is flanked by two seven-storey towers.