 The casemated secondary battery had worth of armor protection, and 8 cm thick gun shields.The "Helgoland"s were also fitted with anti-torpedo nets, but these were removed after 1916.Four ships of the class were ordered, under the provisional names "Ersatz Siegfried" ("Helgoland"), "Ersatz Oldenburg" ("Ostfriesland"), "Ersatz Beowulf" ("Thüringen"), and "Ersatz Frithjof" ("Oldenburg"), as replacements for three of the coastal defense ships of the "Siegfried"-class, and the unique coastal defense ship SMS "Oldenburg".