There were now 96 rifled cannon pieces and 38 mortars in full fire at very short range. Each gun was authorized to fire twenty rounds a day and ten shrapnel each night. The large Finkmatt Barracks were destroyed by fire, and the Stone Gate was so much injured that it had to be buttressed with sandbags. The garrison withdrew the guns behind the parapet, and only fired their mortars. However, in order to push forward the siege-works, sap-rollers had to be brought into use.
When it was discovered that mining galleries were being driven in front of lunette No. 53, Captain Ledebour let himself down by a rope into the ditches, and with the help of his pioneers removed the charges of powder.
During the night of the 13th—14th, the crest of the glacis in front of both the lunettes Nos. 52 and 53 was reached. The crowning was then begun by means of the double traverse sap, and was finished in four days.
The attack henceforth was exclusively directed against bastion No. 11.
To run off the water from the ditches of the fortress it was necessary to destroy the sluices by the Jews' Gate. These were invisible from any part of the field of attack, and the desired result could only be very incompletely obtained by artillery fire at a distance of more than a mile. Detachments of the 34th Fusilier Regiment, therefore, on the 15th, marched on the sluices under a heavy rifle fire from the besieged, and destroyed the dam.
The island of Sporen was at this time taken possession of by the Baden corps.
When the mortar-batteries had for the most part been moved up into the second parallel, the gun-batteries were also advanced nearer, and the wall-piece detachments did such execution by their accurate practice that the defenders never more dared to show themselves by day.
The retaining wall of lunette No. 53 could only be reached by indirect fire; but 1000 shells made a breach, and on the 19th September two mines were fired, which blew up the counterscarp and brought it down to the level of the water of the ditch. The pioneers immediately set about laying a dam of fascines across the ditch. A party sent over in a boat found the work abandoned. The gorge was closed under heavy rifle fire from the ramparts of the main fortress, and the parapet reversed so as to face the place.
The next lunette to the left, No. 52, was merely an earthwork, and the attack had already been pushed forward as far as the edge of the ditch, but earth screens had first to be thrown up and covered in with railway iron, as a protection against the heavy fire of shell from bastion No. 12. The construction of a dam of fascines or earth, more than sixty paces across, and with the ditch full of water almost fathom deep, would have taken a long time; so it was decided to make a cask bridge of beer-barrels, of which a quantity had been found in Schiltigheim. This work was begun at dusk on the 21st, under no better protection than a screen of boards to prevent observation, and it was finished by ten o'clock. Here again the defenders had not waited for the escalade, and this lunette, too, was immediately prepared for being held. Both lunettes were now furnished with batteries of mortars and guns to silence the fire from the ravelines and counter-guards of the front of attack, against which five dismounted and counter-batteries were also directed.
During the night of the 22nd—23rd the Germans advanced from lunette No. 52, partly by flying sap and partly by the deep sap, and there followed the crowning of the glacis in the front of counter-guard No. 51. A breaching fire was immediately opened against the east face of bastion No. 11, and the west face of bastion No. 12. The splinters of stone compelled the defenders to abandon the counter-guards. The scarp of bastion No. 11 fell on the 24th, after a shell-fire of 600 rounds. The bringing down of the earthwork angle which remained standing, was postponed till the beginning of the assault.