Hepburn at once set to work to strengthen the position, to demolish all the houses and walls outside the defences, cut down and destroy all trees and hedges which might shelter an enemy, and to strengthen the walls with banks of earth and platforms of wood. For three days the troops laboured incessantly; on the third night the enemy were heard approaching. The advanced troopers and a half company of infantry were driven in, contesting every foot of the way. When they reached the walls heavy volleys were poured in by the musketeers who lined them upon the approaching enemy, and Tilly, supposing that Gustavus must have moved forward a considerable portion of his army, called off his troops and marched away to Nuremberg. Two days later Hepburn was ordered to return with his force to Wurtzburg. The king now broke up his camp near Wurtzburg, and leaving a garrison in the castle of Marienburg and appointing Marshal Horn to hold Franconia with 8000 men, he marched against Frankfort-on-the-Maine, his troops capturing all the towns and castles on the way, levying contributions, and collecting great booty. Frankfort opened its gates without resistance, and for a short time the army had rest in pleasant quarters. The regiments were reorganized, in some cases two of those which had suffered most being joined into one. Gustavus had lately been strengthened by two more Scottish regiments under Sir Frederick Hamilton and Alexander Master of Forbes, and an English regiment under Captain Austin. He had now thirteen regiments of Scottish infantry, and the other corps of the army were almost entirely officered by Scotchmen. He had five regiments of English and Irish, and had thus eighteen regiments of British infantry. At Frankfort he was joined by the Marquis of Hamilton, who had done splendid service with the troops under his command. He had driven the Imperialists out of Silesia, and marching south, struck such fear into them that Tilly was obliged to weaken his army to send reinforcements to that quarter. By the order of Gustavus he left Silesia and marched to Magdeburg. He had now but 3500 men with him, 2700 having died from pestilence, famine, and disease. He assisted General Banner in blockading the Imperialist garrison of Magdeburg, and his losses by fever and pestilence thinned his troops down to two small regiments; these were incorporated with the force of the Duke of Saxe-Weimar, and the Marquis of Hamilton joined the staff of Gustavus as a simple volunteer.