A watchtower was constructed on the Balberg by the Romans for protection of the road between Castra Vetera (near modern Xanten) and Blerick (Netherlands).

The name of the settlement has two possible origins; the first from Latin "sus" (= pig) and the second from the Low German word "soneman" (= arbiter), so the name means either "Schweinebach" ("pig stream") or the stream where the arbiter resides.

Sonsbeck became a place of pilgrimage, when a chapel was constructed to preserve the relics of the martyr Gerebernus.

After the death of her mother, the king wanted to marry his daughter, wo fled to Geel (Belgium) with her tutor Gerebernus, where they were captured and beheaded.

In the following years, the construction of walls around the city began, they were finished in 1420 and included a keep for the Counts/Dukes of Cleves.