As soon as Gustavus perceived that Nuremberg was the point towards which Wallenstein was moving, he hastened at once from Munich to the assistance of the threatened city. The forces at his disposal had been weakened by the despatch of Marshal Horn to the Lower Palatinate, and by the garrisons left in the Bavarian cities, and he had but 17,000 men disposable to meet the 60,000 with whom Wallenstein was advancing. He did not hesitate, however, but sent off messengers at once to direct the corps in Swabia under General Banner, Prince William of Weimar, and General Ruthven, to join him, if possible, before Nuremberg. Marching with all haste he arrived at Nuremberg before Wallenstein reached it, and prepared at once for the defence of the city. He first called together the principal citizens of Nuremberg and explained to them his position. He showed them that were he to fall back with his army he should be able to effect a junction with the troops under his generals, and would ere long be in a position to offer battle to Wallenstein upon more equal terms, but that were he to do so he would be forced to abandon the city to the vengeance of the Imperialists. He told them that did he remain before the city he must to a great extent be dependent upon them for food and supplies, as he would be beleaguered by Wallenstein, and should be unable to draw food and forage from the surrounding country; he could therefore only maintain himself by the aid of the cordial goodwill and assistance of the citizens. The people of Nuremberg were true to the side they had chosen, and placed the whole of their resources at his disposal. Gustavus at once set his army to work to form a position in which he could confront the overwhelming forces of the enemy. Round the city, at a distance of about thirteen hundred yards from it, he dug a ditch, nowhere less than twelve feet wide and eight deep, but, where most exposed to an attack, eighteen feet wide and twelve deep. Within the circuit of this ditch he erected eight large forts and connected them with a long and thick earthen parapet strengthened with bastions. On the ramparts and forts three hundred cannon, for the most part supplied by the city of Nuremberg, were placed in position. As the camp between the ramparts and the town was traversed by the river Pegnitz numerous bridges were thrown across it, so that the whole force could concentrate on either side in case of attack. So vigorously did the army, assisted by the citizens, labour at these works, that they were completed in fourteen days after Gustavus reached Nuremberg.