The defenders also removed the roof of the St. Michael's Chapel in the castle's outer ward, filled the chapel with dirt to reinforce its walls, and placed some of their artillery pieces within the walls. Ferdinand reluctantly ordered saps to be dug into the side of the mountain. The sapping was difficult and dangerous and the sappers worked under continuous attack from the castle's defenders, who fired on them with small arms and the castle's artillery and dropped rocks and debris on their heads. The forced labor of local peasants minimized losses among Ferdinand's own troops, but many of the peasants perished in the effort. On 6 December, the sappers reached the south-eastern side of the fortress's outermost wall and then spent another ten days undermining the basalt on which the castle stood; they completed their work on 16 December and placed of powder into the mine. Ferdinand reported on the siege's progress in a letter to his older brother, Duke Wilhelm, dated 15 December 1583: "The fortress stands on solid rock. ...[ Y]esterday we had reached the outer wall of the castle, and in a day or two we hope to send the fortress into the sky." On 17 December, Ferdinand again asked the castle's defenders to surrender. They replied that they did not know the meaning of the word and would hold the Godesburg to the last man. A report dated 23 December 1583 relates that, having given Ferdinand a rude reply, the defenders went back to lunch. Ferdinand ordered 400 men to enter the saps; these men would storm the castle once the mine had been detonated. The remainder of his cavalry and foot soldiers were to wait in the fields below. Some sources assert that the fuse was lit at around 1:00 pm, although the 19th-century local historian Heinrich Joseph Floß argued that these sources are mistaken, and that the explosion clearly occurred in the morning. All sources agree that the explosion, with a dreadful crack, propelled chunks of the towers and walls high into the air. Almost half the Godesburg collapsed instantly. According to a newspaper report dated 13 January 1584, debris raining on the valley below damaged several houses, and destroyed some of them completely. Amidst the flames and rubble, Arenberg's and Ferdinand's troops tried to storm the castle, but found their way blocked by masses of debris created by their own explosives. Furthermore, although close to half of the garrison had perished in the explosion and subsequent collapse of the fortifications, those who remained offered staunch resistance by throwing rocks on the approaching attackers, causing a large number of casualties.