At the beginning of the blockade, inundations and marsh-land rendered Schlettstadt, a fortified town of 10,000 inhabitants, unapproachable on the east and south, and partly on the north. The place itself, perfectly safe from storm, with high walls and a wet ditch, was armed with 120 guns, but garrisoned with only 2000 men, for the most part Gardes-Mobiles. There was a deficiency of safe casemates, and on the west front vineyards and hedgerows favoured the near approach of assaults, while the railway embankment was a ready-made protecting wall for the construction of the first parallel. To divert the attention of the besieged from this front of attack, a battery was constructed on the 20th at the Kappel Mill on the south-east, from which fire was opened on the barracks and magazine in the town, and on the sluice which maintained the inundation. By the evening of the 21st, the infantry posts had advanced to within 400 paces of the glacis, and the construction of the first parallel was proceeded with that night, immediately behind the railway, as well as of emplacements for six batteries within 1230 feet from the ramparts. The garrison fired in the dark on the entire field of attack, but almost without effect. By the morning the trenches were two feet wide and three and a half feet deep, and 20 heavy guns and 8 mortars were ready to open fire. A hot artillery duel now began with the fortress, which replied very steadily. The battery at the mill subjected the west front to a telling reverse fire, and several guns and embrasures were severely damaged. The town was fired at several points, and the defenders' fire gradually ceased. During the night, which was very stormy, the batteries of the attack kept up their fire, the parallel was widened and two new batteries were begun.

At daybreak of the 24th the white flag was seen flying, and a capitulation was forthwith signed, by which Schlettstadt surrendered with its garrison and war-material. The commandant begged the Germans to take possession at once, as the greatest disorder reigned within the town. The public buildings were being plundered by the mob and the drunken soldiery, and a powder-magazine was actually on fire. The German battalions promptly restored order, extinguished the flames, and took away the prisoners. Seven thousand stand of arms fell into German hands, besides the fortress artillery and a large quantity of stores. The siege had cost the victors only twenty men. Schlettstadt was occupied by Etappen troops, and the battalions released from duty there marched into southern Alsace, three of them going to strengthen the siege of Breisach, which was now being proceeded with.