The siege of Aachen, which lasted from late April or early May until October 1248, was part of the German civil war that began with Pope Gregory IX's proclamation of a crusade against the Emperor Frederick II in 1240.
[3] When the city of Aachen refused to allow William's supporters in, they began to surround it to besiege it in late April or early May.
Despite the presence of the counts and their retinues, the besieging force was not large enough to completely invest (surround) the city, leaving the defenders a means of supply and communication.
The imperial bailiff of the city and the twelve noble citizens pledged obedience to Pope Innocent and King William and were not punished further.
That of Aachen, however, was the largest military endeavour, requiring enough troops to surround the entire city, most of them kept in the field for six months.
[5] On 3 November, William as king confirmed the rights and freedoms of the West Frisians that had supposedly been granted by his predecessor, Charlemagne, almost 450 years earlier.