Monschau Castle is thought to have been erected in the early 13th century. In the mid-14th century it was expanded and fortified with its massive ring walls and battlements, complete with a system of guard walks. In 1543 Counts of Jülich was besieged by the Emperor Charles V, who conquered it with heavy guns and proceeded to pillage the town. Some repairs were carried out, but only to expand the lower castle battlements.
The French confiscated the castle as part of the national treasure, then sold it. However, the new private owners could not afford to maintain the complex,so around 1836/37 they had removed the roofs to avoid building tax. The castle then fell to ruin,until beginning of 19th century buildings were reinforced, repaired and renewed.As you can see from pic 2.some part of tower are mixing between old brick and new white concrete.
After World War I, a youth hostel was set up in the western wing.