I mean the budget is 125 million. I would start at the creative-end first, writers, directors and then work out production cost/talent. I understand talent sells films and they have plenty of big names like Fassbender and Jeremy Irons but it seems like they did it the other way around. It's like they (and I say they, like I know who it is) tried skimming on the writer/director.
I think that's exactly what they did. Which implies that they don't have any confidence in their own brand. That's sad. When you're confident in your brand you know that you can hire unknown actors, and maybe turn them into future stars. That allows you to focus on screenplay and directing. Kind of like casting all those new actors in The Force Awakens. I realize that Assassin's Creed is nowhere near as famous as Star Wars of course, but it's famous enough that they could have gotten away with casting someone we've never even heard of before.