ENSEMBLE STUDIOS HAD spent nearly a decade mastering the real-time strategy genre when it became a victim of its own success. Its 1997 debut, Age of Empires, had been an instant hit and spawned several beloved sequels and expansions that cemented the studio’s glowing reputation. Imitators—some novel, some slavish—quickly appeared in their droves, and by the mid-2000s, parent company Microsoft expected nothing of the studio other than to build on its legacy with another game in the same vein. The only problem: Ensemble wasn’t much interested. “We had been trying for so long to do anything but an RTS game,”…