The British tried to rally around the Western battery, but the battery's travelling magazine (a portable chest containing cartridges) exploded, apparently as the result of an accident. This caused further loss (including 20 killed) and confusion among the British regulars, and they fell back to a ravine at Garrison Creek north of the fort, where the militia were forming up. American forces advanced east towards the fort, and assembled themselves outside its walls, exchanging artillery fire with the fort. The naval squadron also bombarded the fort, having re-positioned themselves directly south of the fort's stockade. Sheaffe decided that the battle was lost and ordered the regulars to retreat, setting fire to the wooden bridge over the River Don east of the town to thwart pursuit. The militia and several prominent citizens were left ""standing in the street like a parcel of sheep"". Sheaffe instructed the militia to make the best terms they could with the Americans, but without informing the senior militia officers or any official of the legislature, he also dispatched Captain Tito LeLièvre of the Royal Newfoundland to set fire to the sloop of war HMS "Sir Isaac Brock" under construction at York's Naval Shipyard, and to blow up the fort's magazine. The two sides continued to exchange artillery fire until Sheaffe's withdrawal from the fort was complete. The British had left the flag flying over the fort as a ruse, and the Americans assembling outside its walls assumed that the fort was still occupied.