The "Nassau"-class ships were protected with Krupp cemented steel armor. The basic armor layout divided the ships into three sections: the bow, the stern, and the central citadel, the latter extending from the fore to the aft main battery barbette. The citadel consisted of the main section of belt armor, connected at either end by transverse armored bulkheads, and supported by a curved armor deck at mid-deck level. It protected the ships' vitals, including their propulsion machinery spaces and ammunition magazines. On either end of the citadel, the belt was considerably reduced in thickness and the deck was lowered to waterline level forward, though aft it remained at mid-deck level. The need for improved underwater protection had been demonstrated during the Russo-Japanese War of 1904–1905, during which several battleships of both sides had been badly damaged or sunk by naval mines and torpedoes; also of major importance was the severe damage to the battleship in 1901 after having struck an uncharted rock.