When Archduke Franz-Ferdinand of Austro-Hungary and his wife were assassinated in Sarajevo in July, 1914, the whole world started its preparations to and open conflict. Austrians blamed the Serbs for arranging the whole affair and set an ultimatum, which meant an inevitable war in the Balkans. What we know as The Great War, soon became a worldwide struggle of old empires... A stalemate in Serbia resulted in construction of new weapons of war, one of which was the mighty Austrian "Mobilzitadelle", a huge steam- and diesel-powered fortress on wheels, capable of destroying a city within a few days.
The day is 15th July, 1916. This is the day when the Zitadelle was assaulted, immobilized and successfully destroyed by Serbian soldiers, despite its fanatic defence conducted by both Austrians and Prussians...
Old idea, but I only had a rough sketch of it. I finished it in the last few weeks. HB pencil on A4.