The ground on the Swedish bank of the river was higher than that facing it; and when the Swedish batteries opened they so completely swept the ground inclosed by the curve of the river that the Imperialists could not advance across it, and were compelled to remain behind a rivulet called the Ach, a short distance in the rear of the Lech. They brought up their artillery, however, and replied to the cannonade of the Swedes. For four days the artillery duel continued, and while it was going on a considerable number of troops were at work in the village of Oberndorf, which lay in a declivity near the river, hidden from the sight of the Imperialists, constructing a bridge. For that purpose a number of strong wooden trestles of various heights and with feet of unequal length for standing in the bed of the river were prepared, together with a quantity of piles to be driven in among and beside them to enable them to resist the force of the current. On the night of the fourth day the king caused a number of fires to be lighted near the river, fed with green wood and damp straw. A favourable wind blew the smoke towards the enemy, and thus concealed the ground from them. At daybreak on the 5th of April, a thousand picked men crossed the river in two boats, and having reached the other side at once proceeded to throw up intrenchments to cover the head of the bridge, while at the same time the workmen began to place the trestles in position. As soon as day broke Tilly became aware of what was being done, and two batteries opened fire upon the work at the head of the bridge and against the bridge itself; but the low and swampy nature of the ground on the Imperialist side of the river prevented his placing the batteries in a position from which they could command the works, and their fire proved ineffective in preventing the construction of the bridge. Seeing this, Tilly at once commenced preparations for arresting the further advance of the Swedes. To reach his position they would be obliged to cross the swampy ground exposed to the fire of his troops, and to render their progress still more difficult he proceeded to cut down large trees, lopping and sharpening their branches to form a chevaux-de-frise before his troops. All the morning a heavy cannonade was kept up on both sides, but by noon the bridge was completed and the advance guard of the Swedes, led by Colonels Wrandel and Gassion, advanced across it. As the other brigades were following, Tilly directed General Altringer to lead his cavalry against them.