The former castle was surrounded by the Erft and two moats. It already existed before the old town of Kaster was surrounded by a castle wall. Its location was favorable because one of the few crossings over the Erft was possible there. The Erft also often demarcated the area of Cologne from that of Jülich. For a time, Kaster also served as a customs post.
In front of the main castle, the current ruin, was a bailey. It is believed that this is where the urban development of Kaster came from. In Welser's drawing you can still see the rectangle of the outer bailey. The Eulenturm, which is still standing today, was one of these four corner towers. In the outer bailey was the waiter's shop with storage buildings and handicraft workshops, even a mouse catcher from Maastricht was employed there. From there the waiters (cellarius) administered the payments in kind and money to the feudal lords.
The lords of Caster (known from 1148) were the feudal lords of the place and probably owned the castle. After their extinction, Kaster went back to the Counts of Jülich, who now expanded Kaster into a Jülich office. The castle was first mentioned in writing in 1273 when it was sold by the Count of Jülich to King Rudolph of Habsburg. He gave it back to Count Wilhelm IV of Jülich as a fief.
Source:
geschichtsverein-bedburg.de/permanent/KasterBurg.html