The fortress of Ossowetz on the Bobr River proved inconquerable by the 42-centimeter mortars which had worked such terrific effects on the forts of Belgium and France. It was continually under German artillery fire through the months of February and March, 1915, without suffering appreciable damage. The great mortars were brought up within range of the fortress with much difficulty, owing to the fact that the place is almost completely surrounded by swamps. The Germans apparently had counted seriously at first on making a breach in the Russian defensive lines at this place. After persistent attempts to make an impression on the fortress with their heaviest guns they were obliged, however, to content themselves with keeping the garrison in check so as to forestall offensive moves. A German artillery officer who took part in the bombardment relates that the chief obstacle to the pressing home of an attack were several heavily armored batteries which lay concealed outside the visible works of the fortress itself in the broad strip of swampland surrounding it. These were built deep into the ground, protected by thick earthworks, and very effectively screened from observation. They were a constant menace and apparently could not be destroyed by the German fire. Even though the main fort itself had been destroyed they would have prevented the approach of the enemy's troops, for they commanded the only causeway leading through the swamps to the fortress and would have blown to pieces any infantry that ventured to push along this road. Furthermore, even the intense cold did not make the swamp passable except by the roadway because warm springs here and there prevented the ice from freezing sufficiently strong to bear the troops. The German gunners noted too that their shots fell practically without effect, plunging quietly into the mud to a great depth so that they did not even throw up earth or mud. The result was that the 42-centimeter monsters were hastily withdrawn after a few trial shots and the bombardment was continued with a battery of 28-centimeter coast defense guns, an Austrian motor battery, a 30.5-centimeter mortar and some other heavy batteries. The fire rose to considerable intensity in the last days of February and the first days of March.