This CGI breakdown of Game of Thrones' Hardhome shows how painstaking the battle sequence was to make

The massacre at green screen

Visual effects are now so advanced that it's often difficult to spot what it practical effects and what is CGI, particularly when there is as much going on in the shot as there was during Game of Thrones' fantastically arranged 'Hardhome' battle scene.

El Ranchito, the digital postproduction company behind its VFX elements, has posted a video breaking down how it was put together, edited in such a way that the action still flows while showing you all the different elements that had to be altered to achieve the scenes of absolute chaos.

Seeing how the undeads skulls were crushed is particularly interesting, as is the way Kit Harrington convincingly slays enemies that weren't even there.

All in all, the 20-minute sequence took a "solid month" to film.

"We spent a month filming that sequence which was 20 minutes long," Harington told E!. "A month, a solid month. It equated to about a minute of footage a day."

"It was heavy and it was important to me that it worked because it's a really important beat in the Game of Thrones story. I thought it was brilliant. I really, really loved it. I was so proud of it and happy with it. I never pat myself on the back but I thought, why not? It was great!"

The episode was directed by Miguel Sapochnik, contributing only his second episode to the show so far. He will be back for season 6, directing episodes nine and 10.

@christophhooton