I don't want to spoil things for you, but the epic battle at Hardhome, easily the highlight of Game of Thrones' fifth season, wasn't filmed using real zombies. Here's how the scenes were actually shot.
SPOILERS BELOW AND IN THE VIDEO
Visual effects company El Ranchito, who worked on the episode, have posted a VFX breakdown of all the tweaks and additions they had to make to the actual set (which was shot in an old quarry in Northern Ireland) to get it looking suitably apocalyptic.
Given the quarry featured no ships, giants or walking dead, they had some work to do.