Game of Thrones season 8, episode 3: Why was the Battle of Winterfell so dark?

GAME OF THRONES season 8, episode 3 - The Long Night - was the longest instalment of the HBO series ever and the most cinematic one of the lot. But many Game of Thrones fans have criticised programme-makers for how visually dark the episode was and how many were unable to watch the action properly. So why was the Battle of Winterfell and The Long Night so dark?