If you intend to visit Jülich, there is one place you have to go in any event: The Citadel Fortress. It is a historic masterpiece designed and built by the Italian architect Alessandro Pasqualini, famous since 1530 as an outstanding military and civilian architect in the Netherlands. Contruction started in 1548 onwards under Duke William V, representing both modern fortifications and also with the ducal palace the start and at the same time a high point of Italian Renaissance architecture in the Rhineland.
Today, the building serves as high school - I myself studied there from 1990-1999.
The official city website has a great historic summary written in English.
The Witches Tower ("Hexenturm" in German) is the town´s landmark and entrance to the pedestrian precinct. It was the town gate (Rur Gate) of Jülich's medieval town wall until 1547, of which a small section still remains in a backyard between Poststrasse and Stiftsherrenstrasse, accessable from there. It was built in the early 14th century after the capture and destruction of Jülich by Archbishop Siegfried of Cologne in 1278, which revealed the inadequacy of the previous fortifications; wall thickness up to 2.30m, street level now about 1 m above the original state. A small projection for the privy can be seen on the northern tower next to the former adjacent town wall, which was 1.70 m thick.
The Hexenturm had four levels of defence, including the originally flat, crenellated roof. The present shape of the roof dates from the 17th century. After demolition of the town wall at the beginning of the modern period, the Hexenturm was used as a jail and torture chamber for the main and criminal courts of the duchy. Witch hunts were extremely rare in Jülich; as early as 1563, the Duke's personal physician, Johannes Weyer, opposed the obsessive belief in witches widespread in Europe at that time with his book "De praestigiis daemonum", which was immediately placed on the Index.
Two stones originating from Roman tombs (late first century AD) are incorporated into the outer masonry on the town side: one of them has a representation of a funeral repast and the other a man wearing a toga (illustrating his status as a Roman citizen).
Jülich's local history museum (Stadtgeschichtliches Museum) with an interesting permanent collection an also special exhibitions is housed in the "Kulturhaus am Hexenturm", whose west front reflects the medieval town wall in architecture and course.
The Brückenkopf Park is a wonderful recreation area in Jülich. In 1998 they had a gardening/ landscaping exhibition here and kept the grounds as a park packed with sports facilities (soccer, volleyball, climbing), playgrounds, mini golf, a little zoo, picnic areas, restaurants and some historical builings, the Brückenkopf. These were defence builidings of the French, built in the beginning of the 19th century.