Epic’s free-to-play battle royale title defies all conventional wisdom about crossover gaming success.Fortnite, a free-to-play shooter by Epic Games (Gears of War), has taken over the world. That may sound like exaggeration, but I couldn’t overstate the popularity of Fortnite if I tried. It is massively played, and even more massively watched—on Twitch, 66 million hours of Fortnitehave been watched in the past two weeks, with about 200,000 viewers tuning in at any given time. (In March, the streamer Ninja played with Drake, raking in the highest single-game viewership Twitch has ever seen.) It is also, somehow, massively complicated.