In 1672, Maastricht was a key position in the Dutch defensive strategy. They had already noticed that the French were preparing forward supply bases in the Bishopric of Liège for a swift advance to the north. The Dutch concentrated a large part of the mercenary troops available to them in Maastricht, some eleven thousand men, hoping that a prolonged siege would gain them sufficient time to prepare the six Rhine fortresses defending the eastern border of the Republic. The French reached Maastricht on 17 May 1672 but, on advice of Turenne, avoided a direct assault, instead occupying the satellite forts of Tongeren, Maaseik and Valkenburg. They thus basically by-passed Maastricht to quickly overrun the Rhine fortresses, leading to a collapse of the Dutch defences. They maintained only a small observation force, a few thousand men, near the fortress. Not capturing Maastricht had its draw-backs because the fortress threatened the extended French supply lines. In November 1672, stadtholder William III of Orange advanced from Holland to Maastricht, taking part of its garrison with him, and he continued through the Spanish Netherlands to attack Charleroi, the French city at the start of the supply route.