During World War I, the complex was the largest defensive structure in the entire state, occupying 65 km2 (25 sq mi).

[1] The fortress was battle-tested in 1915 when Germany attacked the Russian Empire, and withstood eleven days of assault before capture.

After World War I, the fortress' military importance declined as advances in weaponry rendered it increasingly obsolete.

[2] During World War II, parts of the fortress complex were used by the Nazi Germany for detention, interrogation, and execution.

[3] Some sections have since been restored; the Ninth Fort houses a museum and memorial devoted to the Jewish victims of Holocaust mass executions.

[4] The city of Kaunas is located at the confluence of two rivers, the Neman and Neris, which link Lithuania's interior and its capital, Vilnius, to the Baltic Sea.

[5] Since Lithuania was heavily wooded and its lands were often impassable, its interior was most approachable along its rivers when frozen and during the short dry harvest season in late summer.

[6] In response to this vulnerability, defensive structures, including a brick castle in Kaunas, were in place at various points on the Nemunas River by the 14th century.

[6][7] The city was first mentioned in written sources in 1361; it received Magdeburg rights, regulating its autonomy and establishing trade protocols, from Vytautas the Great in 1408.

[7][8][9] Russia's western borders needed support, and fortresses existed or were being built in Latvia, Ukraine, and Belarus.

The concept of building a fortress in Lithuania was discussed without result in 1796,[10] but became a critical concern after the French invasion of Russia in 1812 led by Napoleon.

Facing this possibility and evaluating the natural advantages of the city, Russian officials decided to construct a fortress there.

[15] As originally planned, the fortress encompassed a huge site, consisting of seven forts and nine defensive batteries arranged in concentric loops.

"[17][18] The first forts were built using bricks reinforced with thick ramparts of earth, which were incorporated into the surrounding relief, making them harder to breach.

[15] Batteries were built between adjacent forts; these were fortifications containing various types of artillery, located along the fortress' outer lines and usually erected on the hills.

[16][19] The fortress was designated first-class in that year, marking its importance and defensive capabilities, and Otto Klem was named its first commandant.

[15] At the same time, massive groundworks were laid along with additional defensive structures, effecting the complete enclosure of the city center between the Rivers Nemunas and Neris.

[16] As new building and weapons technologies developed, the fortress was repeatedly renovated in order to maintain its military effectiveness.

[23] Adjustments to the secret protocols of the 1939 Molotov–Ribbentrop Pact assigned Lithuania to the Soviet sphere of influence, and it was occupied by the USSR in June 1940.

On July 6, acting under orders of the SS, Lithuanian auxiliary police units shot nearly 3,000 Jews at the Seventh Fort.

On October 28, the "Great Action" took place—the residents of the Kaunas Ghetto were summoned, and over 9,000 men, women and children were taken to the Ninth Fort and executed.

[32] In 2005, the international project "Baltic Culture and Tourism Route Fortresses" was launched, with support from the European Union.

Its goal is the promotion of transnational scientific cooperation in monument protection, along with the creation of strategies to reconstruct and manage fortresses in the region.

[18] Other restoration issues include uncovered wells, poor drainage and ventilation, erosion, possible chemical contaminants, vegetative overgrowth, and the presence of a protected bat colony.