Dawn of Man

You unlock this by completing all 30 milestones in the 3 scenarios in Freeplay. At first only Continental Dawn is available. The Northlands requires 5 unlocked milestones, and Ancient Warriors requires 10 unlocked milestones to access. You could play on hardcore difficulty and combine it with the 2 achievements related to completing milestones on the hardest difficulty. I just played on normal to learn how to play the game. The milestones are the same for both difficulty settings.