This episode required a heavy budget for CGI, and it got to the point where the visual effects crew had to ask HBO for more money.
The battle sequence at Hardhome was initially scripted as being much smaller in scope, and was to be filmed in Iceland, like all scenes that take place beyond the Wall. However, the scene was then extensively rewritten as a major battle scene relatively late in season 5's production schedule. Since the remaining window for filming in Iceland during the winter had become too small, a fully realized set (including buildings and a 300 ft long, 18 ft high wooden palisade) was built at Magheramorne Quarry in Northern Ireland, the same quarry where the Castle Black set was built. Background plates were actually shot in Iceland, and later digitally pasted in. About 400 extras were trained in fighting, after which the 220 best were used for filming, together with 50 stuntmen.
Kit Harington (Jon Snow) said this episode took "three to four weeks of shooting for a sequence that should add up to 20 minutes." He added: "[The Battle of Castle Black in Season 4, Episode 9] last year was the hardest thing I've ever done. This [episode] was twice as hard as that. It was exhausting."
The language Wun Wun speaks in to Eddison Tollett is the Old Tongue of the First Men, a language brought to Westeros over 12,000 years prior. The language is almost extinct except beyond the Wall, since most of Westeros, including the North, has taken over the Andal language (also known as the Common Tongue) over the millennia. Only the most isolated folks beyond the Wall who had limited contact with the Andals, such as the Giants and Thenns, still speak a rudimentary form of the Old Tongue.
Fifty stunt performers and 220 extras worked on the Hardhome sequences for a sixteen day filming period over three weeks in a cold, rainy November. The mantra on set used to cope with the grueling filming conditions was "Go hard or go home."