The Russians likewise claimed advantages from their forward thrust from Grodno. As represented by the Russian General Staff the withdrawal of the Germans from a front close to the line of the fortress in the first place was not a voluntary one, as it is pictured in the German account, but was forced by the strong pressure exerted by the Russian attacks following upon their retreat after the "winter battle." Thus they report the complete defeat of two German army corps, resulting in the seizure by the Russians of Height 100.3, which they described as dominating the entire region of the operations before Grodno. "In this battle," says the Russian report of March 5, 1915, "we took 1,000 prisoners and six cannon and a machine gun. Height 100.3 was defended by the Twenty-first Corps, the best of them all which lost during the battle 12,000 to 15,000 soldiers, as can be estimated from the dead left behind. After the shattering of the German counterattack at Height 100.3 the operations of the enemy became entirely passive. We, on the other hand, took village after village, and everywhere made prisoners." 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.