The quick advancement of artillery technology beginning in the 1850s provoked an arms race between artillery and military architecture.
Rifled artillery could now fire out of range of fortress guns, so military architects began placing forts in rings around cities or in barriers to block approaching armies.
These forts were vulnerable to new artillery shells, which could penetrate earth to destroy masonry underground.
In response, star forts evolved into polygonal forts, mostly underground and made of concrete with guns mounted in armoured, rotating casemates.
Combining rings and barriers, France created a vast fortified zone on their border with Germany, while Belgium began construction of the National Redoubt in 1888.

The German Empire also fortified its borders, but Chief of the General Staff Helmuth von Moltke the Elder desired the ability to break through Franco-Belgian fortifications.
Although German artillery had been effective during the Franco-Prussian War, it had been allowed to stagnate.
By the 1880s the barrel diameter of the German Army's most powerful gun, the 21-centimetre (8.3 in) field howitzer, was no longer adequate against fortresses.
Moltke began requesting more powerful guns that same decade.
More powerful artillery became essential to his successor, Alfred von Schlieffen, who planned to quickly defeat France by sweeping through Belgium in response to the 1893 Franco-Russian Alliance.
To be able to reduce French and Belgian fortresses, the Artillerieprüfungskommission (Artillery Test Commission, APK) formed a partnership with Krupp AG in 1893.
The first result of this partnership was a 30.5 cm (12.0 in) mortar, accepted into service four years later and designated the schwerer Küstenmörser L/8, or the Beta-Gerät, to disguise its purpose as a siege gun.
Tests in the mid-1890s showed that the Beta-Gerät could not destroy French or Belgian forts, even with improved shells.
Interest in a more powerful siege gun waned until the Russo-Japanese War, during which the Japanese Army used 28 cm (11 in) coastal guns brought from Japan to end the 11-month long Siege of Port Arthur.

In 1906, Helmuth von Moltke the Younger became Chief of the General Staff and instructed the APK to study and improve the performance of the Beta-Gerät.
The APK recommended a more powerful gun, perhaps as large as 45 cm (18 in), but the Germany Army opted for a 30.5 cm howitzer, the Beta-Gerät 09, and a 42 cm (17 in) gun.
Design and testing for the Gamma-Gerät began in 1906 and lasted until 1911.
Although the Gamma-Gerät had the destructive power the General Staff required and could outrange French and Belgian fort guns, it could only be emplaced near rail lines and took 24 hours to fully prepare.
As early as 1907, Krupp began development of siege artillery transported by gun carriage.
Testing resulted in a 28 cm (11 in) howitzer transportable over road and countryside, but it was rejected by the APK, as was Krupp's 30.5 cm model.
Finally, in Autumn 1911, Krupp and the APK developed a wheeled 42 cm howitzer, which was designated the 42 cm kurze Marinekanone 14 L/12, or Minenwerfer-Gerät (M-Gerät).
The APK ordered its first M-Gerät in July 1912 and another in February 1913.
Tests of the gun's mobility began in December 1913 and found that gasoline tractors were best for pulling the M-Gerät.
Test firing, at one point observed personally by Kaiser Wilhelm II, began in February 1914, and Krupp estimated that the M-Gerät would be fully completed by October 1914.
The quick advancement of artillery technology beginning in the 1850s provoked an arms race between artillery and military architecture. Rifled artillery could now fire out of range of fortress guns, so military architects began placing forts in rings around cities or in barriers to block approaching armies. These forts were vulnerable to new artillery shells, which could penetrate earth to destroy masonry underground. In response, star forts evolved into polygonal forts, mostly underground and made of concrete with guns mounted in armoured, rotating casemates. Combining rings and barriers, France created a vast fortified zone on their border with Germany, while Belgium began construction of the National Redoubt in 1888.
The German Empire also fortified its borders, but Chief of the General Staff Helmuth von Moltke the Elder desired the ability to break through Franco-Belgian fortifications. Although German artillery had been effective during the Franco-Prussian War, it had been allowed to stagnate. By the 1880s the barrel diameter of the German Army's most powerful gun, the 21-centimetre (8.3 in) field howitzer, was no longer adequate against fortresses. Moltke began requesting more powerful guns that same decade. More powerful artillery became essential to his successor, Alfred von Schlieffen, who planned to quickly defeat France by sweeping through Belgium in response to the 1893 Franco-Russian Alliance. To be able to reduce French and Belgian fortresses, the Artillerieprüfungskommission (Artillery Test Commission, APK) formed a partnership with Krupp AG in 1893. The first result of this partnership was a 30.5 cm (12.0 in) mortar, accepted into service four years later and designated the schwerer Küstenmörser L/8, or the Beta-Gerät, to disguise its purpose as a siege gun. Tests in the mid-1890s showed that the Beta-Gerät could not destroy French or Belgian forts, even with improved shells. Interest in a more powerful siege gun waned until the Russo-Japanese War, during which the Japanese Army used 28 cm (11 in) coastal guns brought from Japan to end the 11-month long Siege of Port Arthur.
In 1906, Helmuth von Moltke the Younger became Chief of the General Staff and instructed the APK to study and improve the performance of the Beta-Gerät. The APK recommended a more powerful gun, perhaps as large as 45 cm (18 in), but the Germany Army opted for a 30.5 cm howitzer, the Beta-Gerät 09, and a 42 cm (17 in) gun. Design and testing for the Gamma-Gerät began in 1906 and lasted until 1911. Although the Gamma-Gerät had the destructive power the General Staff required and could outrange French and Belgian fort guns, it could only be emplaced near rail lines and took 24 hours to fully prepare. As early as 1907, Krupp began development of siege artillery transported by gun carriage. Testing resulted in a 28 cm (11 in) howitzer transportable over road and countryside, but it was rejected by the APK, as was Krupp's 30.5 cm model. Finally, in Autumn 1911, Krupp and the APK developed a wheeled 42 cm howitzer, which was designated the 42 cm kurze Marinekanone 14 L/12, or Minenwerfer-Gerät (M-Gerät). The APK ordered its first M-Gerät in July 1912 and another in February 1913. Tests of the gun's mobility began in December 1913 and found that gasoline tractors were best for pulling the M-Gerät. Test firing, at one point observed personally by Kaiser Wilhelm II, began in February 1914, and Krupp estimated that the M-Gerät would be fully completed by October 1914.
Big Bertha (howitzer)