The design of the 135 mm mortar was influenced by the German Minenwerfers, the heavy trench mortars of World War One. The idea behind the 135 mm heavy mortar was the bombardment of the enemy trenches. The 21 kg heavy shells had a large explosive content and used the mine effect: they penetrate deep in the ground and used the resistance of the earth to reinforce the blast effect. The majority of the 135 mm mortars was installed in retractable turrets, but nine of them were mounted on a casemate mounting shown here. For more information click here.