As an imperial city, Aachen held certain political advantages that allowed it to remain independent of the troubles of Europe for many years. It remained a direct vassal of the Holy Roman Empire throughout most of the Middle Ages. It also was the site of many important church councils. These included the Council of 836, and the Council of 1166, a council convened by the antipope Paschal III. From the early 16th century, Aachen started losing its power and influence. It started with the crowning of emperors occurring not in Aachen but in Frankfurt, followed by the religious wars, and the great fire of 1656 It then culminated in 1794, when the French, led by General Charles Dumouriez, occupied Aachen. On 9 Feb. 1801, the Peace of Luneville removed the ownership of Aachen and the entire "left bank" of the Rhine from Germany and granted it to France.In 1815, control of the town was passed to Prussia, by an act that was passed by the Congress of Vienna. The third congress took place in 1818 to decide the fate of occupied Napoleonic France.
Aachen became attractive as a spa by the middle of the 17th century, not so much because of the effects of the hot springs on the health of its visitors but because Aachen was then — and remained well into the 19th century — a place of high-level prostitution in Europe. Traces of this hidden agenda of the city's history is found in the 18th-century guidebooks to Aachen as well as to the other spas; the main indication for visiting patients, ironically, was syphilis; only by the end of the 19th century had rheuma become the most important object of cures at Aachen and Burtscheid.