 The Derfflinger @-@ class ships were protected with <unk> cemented steel armor , as was the standard for German warships of the period . They had an armor belt that was 300 mm ( 12 in ) thick in the central citadel of the ship , where the most important parts of the ship were . This included the ammunition magazines and the machinery spaces . The belt was reduced in less critical areas , to 120 mm ( 4 @.@ 7 in ) forward and 100 mm ( 3 @.@ 9 in ) aft . The belt tapered down to 30 mm ( 1 @.@ 2 in ) at the bow , though the stern was not protected by armor at all . A 45 mm ( 1 @.@ 8 in ) thick torpedo bulkhead ran the length of the hull , several meters behind the main belt . The main armored deck ranged in thickness from 30 mm thick in less important areas , to 80 mm ( 3 @.@ 1 in ) in the sections that covered the more critical areas of the ship . 
