1. That was an actual wall of cavalry charging at Kit Harington (Jon Snow), not CGI.
Production used 40 horses to film that shot, and they actually charged straight at Harington, stopping at the last second. “We’re a bit annoyed because I think everyone’s going to think it was CGI,” Harington said, “but it wasn’t.”
2. The horse master, Camilla Naprous, was responsible for over 80 horses.
3. The big clash used actual actors and horses charging past Jon Snow.
Production shot about a dozen horses charging at each other and pulling up at the very last minute to make the clash look as real as possible. The rest was filled in with CGI.
4. A remote-controlled camera on a long rig attached to a moving car — was used to film Jon Snow’s charge to rescue Rickon.
They used a Land Rover, because the terrain got muddy quickly.
5. The production design team dressed every fake human and horse corpses in the proper house sigils and armor.
Production designer Deborah Riley said each individual prop body was dressed according to which house they belonged to before being added to the pile. According to Riley, that level of detail made the sight of the piles “deeply moving” even though they were fake.