It was in the shape of a star, and formed of a palisade of trunks of trees set upright in the ground, hewn flat on both sides, and closely fitted together--an excellent defence against musketry, but worthless against artillery.

The garrison of the fort, 370 in number, had eight small cannon and a mortar, with which, all next day, they kept up a brisk fire against the battery which the French were throwing up, and arming with twenty-six pieces of heavy artillery.
Colonel Mercer, the commandant of Oswego, saw at once that the French artillery would, as soon as they opened fire, blow the stockade into pieces, and thinking it better to lose the fort, alone, than the fort and its garrison, he sent boats across the river after nightfall, and the garrison, having spiked their guns, and thrown their ammunition into the well, crossed the river, unperceived by the French.
But Oswego was in no position for defence.

Fort Pepperell stood on the mouth of the river, facing Fort Ontario.

Towards the west and south the place was protected by an outer line of earthworks, mounted with cannon, but the side facing the river was wholly exposed, in the belief that Fort Ontario would prevent any attack in this direction.
Montcalm lost no time.

The next evening, his whole force set to work throwing up a battery, at the edge of the rising ground on which Fort Ontario stood, and, by daybreak, twenty heavy guns were in position, and at once opened fire.