Castles of Steel is a work of non-fiction by Pulitzer Prize-winner Robert K. Massie. It details the naval actions of the First World War with an emphasis on those of the United Kingdom and Imperial Germany. This book continues the examination of the naval arms race between these two powers in Massie's tome Dreadnought: Britain, Germany, and the coming of the Great War. The term "castles of steel" was coined by First Lord of the Admiralty Winston Churchill to illustrate the defensive power and importance of the Grand Fleet.
Other chapters describe German submarine warfare and events triggering America's entry into the war. There are also chapters dedicated to central personalities such as British Admirals John Jellicoe and David Beatty and the German Admirals Franz von Hipper and Reinhard Scheer. The book ends with an account of the scuttling of the German High Seas Fleet in Scapa Flow.
A number of maps and photographs are included.