There was interest in his ideas but they remained untested; this changed with the outbreak of the 1688-1697 Nine Years War which placed great emphasis on manoeuvre and siege warfare. Van Coehoorn was present at the capture of Kaiserswerth and Bonn in 1690; his exact role is unclear but Frederick of Prussia was impressed enough to offer him a position as Major-General in his army. He refused and in 1691 William appointed him commander of Namur where he was finally able to implement his ideas on defensive strategy. Namur was divided into the 'City' on the flat northern bank of the River Sambre and the Citadel on high ground to the south controlling access to the Sambre and Meuse rivers. Van Coehoorn strengthened the 'inner' Citadel with new outworks at Fort William and La Casotte but did not have time to do the same for the 'outer' City area. His garrison of 5,000  was also too small for the active defence he had planned, many being poorly-trained Spanish troops with little interest in fighting for the Dutch.
Question: When did Van Coehoorn refused Frederick of Prussia the position he offered him?
Answer: 1690