The CS department has 6 faculty plus assistants, and 1400 majors. Another 5 years' work gets you a Habilitation degree, which is almost a requirement to be considered for an associate or full professor position. Hiring someone at a German university is a drawn out process. Lists of candidates generated in the department pass up the university hierarchy to the appropriate state ministry, which then sits on them a few months before issuing the offer letter. Once someone has an offer, there is no time limit in which they have to give a yes or no answer.
Some people hold onto their offers over a year while negotiating.
There are some funny rules that work against the applicant. You cannot have more than one offer in hand at a given rank, and you can't change universities after age 52 because of pension considerations. Aachen was the city from which Charlemagne ruled Europe. He defeated the Saracens at Poitiers in 732. He managed to use the favor this earned him with the nobles to get his son, Pippin the Short, placed on the throne.