In a general way five railroad lines have been built through various parts of the Pripet Marshes; the most important being a section of the Rovno-Vilna railroad; two others of special importance to the Russian retreat were the Brest-Litovsk-Pinsk-Gomel and the Ivangorod-Lublin-Cholm-Kovel-Kieff road. The Brest-Litovsk-Minsk railroad also passes in its greatest part through the outlying sections of the Pripet Marshes. The effect of these swamp lands on the Russian retreat and the German advance, of course, was twofold: it increased the difficulty of the Russian retreat, throwing at the same time very serious obstacles in the way of the advancing Germans. To the southward, and in a region very similar in all its characteristics, is the Volhynian triangle of fortresses: Lutsk, Dubno, and Rovno. Here too, during the fighting around these three places, the Russian and German armies had to contend with tremendous difficulties, which were caused chiefly by the fact that this part of the country, with the exception of a few sections, was almost impassable. This fact, undoubtedly, was primarily responsible for the decision of the Russian Government to locate these three powerful fortresses at that particular point, because the very difficulties which nature had provided became valuable aids to a strong defense against an invasion of Russian territory by Austro-Hungarian troops from the south. The fortresses of Lutsk and Dubno date with their beginning as far back as 1878, at which time they were built according to the plans of the Russian General Todleben. A little later the fortifications of Rovno were added to this group, and one of the strongest triangles of Russia's fortifications was formed thereby. The sides of this triangle measure thirty, twenty-five, and forty miles respectively. The longest of these is the line between Lutsk and Rovno, with its back toward the Pripet Marshes. Of the three fortresses Rovno is the most important from a strategical point of view, for it defends the junction of three of the most valuable railroads, the railway leading from Lemberg into Volhynia, that running south from Vilna into Galicia, and the railroad which by way of Berticheff indirectly connects Kieff with both Warsaw and Brest-Litovsk. The three fortresses, therefore, acted as a wedge between the most southeastern and the Polish zones of operations. They secured the connection of any Russian forces in Poland with the interior of Russia, and made possible the transfer of forces through the protection which they gave to the various railroad lines necessary for such a transfer. On account of the conditions of the surrounding territory it was impossible for any attacking army to dispose of the fortresses by investing them with part of their available forces while the balance of them continued on their advance; for the only way to reach the country in back of the three fortresses was by way of the fortresses