Leviathan

"Leviathan is a 2009 novel written by Scott Westerfeld and illustrated by Keith Thompson. First of a trilogy set in alternative version of World War I, it has Central Powers (known in-universe as "Clankers") using mechanized war machines opposed by Entente Powers (as "Darwinists") who fabricate living creatures genetically. The central protagonists are Aleksander, son of Archduke Franz Ferdinand; and Deryn, a Scottish girl with dreams of joining the British Air Service with her brother. The sequels are Behemoth and Goliath.[1] The first two chapters of this book were released with Bogus to Bubbly: Insider's Guide to the World of Uglies." - Wikipedia
The Intro to the Campaign - Chapter 0 - An Angel touched me - takes place in this simulation.