Creating the production design for a movie is an unbelieveable amount of work. Turning a handful of pictures and concept art into sets takes a clear vision, an able staff and a ton of money. Even with that though, the result can end up looking like the Mario Bros movie. With Assassins Creed, the filmmakers dont have to worry about making a plumber break bricks with his head or shoot fireballs out of his hand. Despite some sci-fi elements, Assassins Creed (no matter where/when it is set) is essentially a period piece, and Hollywood knows how to do those. Not that recreating historical locations isnt a ton of work (especially on the sweeping scale an Assassin's Creed movie will likely need), but the production designers will be able to look at classical paintings and sketches for inspiration, probably some of the very same references the developers at Ubisoft used.