Toruń Fortress (Festung Thorn), built from 1872–1894 by the Kingdom of Prussia and located in Toruń (now Poland), is one of the largest fortresses in Central and Eastern Europe. The fortress complex – a chain of forts surrounding the city, as well as numerous smaller fortifications supplementing it – was intended to defend the eastern border of Prussia (with the Russian Empire). Despite much planning and investment the fortress did not play a significant role in the First World War nor in any later conflict.
Toruń (German: Thorn) was an important town located just north of the border between Prussia and Russia, which runs from southwest to northeast, in the north along the Drwęca tributary of the Vistula River which passes through Toruń. With improvements in artillery techniques, including the introduction of rifled barrels and smokeless powder), the old city walls dating from the 18th century no longer provided sufficient protection for the town.
The Prussian government started the construction of fortifications in 1872, at first using French prisoners of war taken during the Franco-Prussian War. The goal was to build a chain of forts surrounding the town of Toruń. Initially five main forts and two medium-size ones were to be built. In the period of 1877–1884, forts II and XI were constructed, followed by forts IV, V, VII, XV, XIII and IX. Continuing changes in artillery and siege techniques, especially the introduction of an anti-building shell in 1883, rendered some of the plans for the fortress obsolete even before it was finished. For that reason, the importance of the main artillery forts was lessened in favor of infantry forts; some artillery forts were redesigned into infantry forts and more small fortifications were constructed. Over time the number of objects to be built increased. In the years 1888–1893, forts III, VI, VIII, X, XII and XIV were added. The last to be constructed was fort I, which was the most technically advanced one. Within 32 years, about 200 fortifications were constructed. Primarily the fortress consisted of seven main forts, six medium-sized ones, six artillery batteries, 32 infantry shelters, and 52 mid-field shelters (used for artillery and ammunition sheltering).