I don't know the film and don't know much about Russian history, so I've no idea how much of the story was original. But I loved it.
The characters were IC and recognisable even in this AU. Your characterisation of Merlin, with his quick thinking and good plans was perfect (those characteristics of him are too overlooked in most fics). The details were just enough to create the atmosphere and make it easy to imagine, and the action sequences were suspenseful. Impressive fic.