Louis, accompanied by members of his court, joined his army in the Spanish Netherlands to take control of the armies in theatre, arriving at the front on 21 March. The King's besieging army of 46,000 (under the direction of his great engineer, Vauban) surrounded the town and its garrison of some 4,800 men. The Allies had formed an army of 38,000 under William to relieve the city, but Luxembourg's army of observation, also 46,000 strong, denied the Allies any possibility of disrupting the operation.