Game of Thrones’ official website makinggameofthrones.com gives us an inside look on those dragon skulls we saw in the Red Keep during the second episode (Stormborn) of season 7. According to the website:
The structures, first created as 3D sculptures by the Game of Thrones art department, were then made into polystyrene carvings by the show’s construction department. Head sculptor Darren Fitzsimons said it took six weeks to finish carving just the largest of the skulls — and that’s before painting and plastering. The massive Balerion the Dread skull is “about 30 feet long, 11 feet high and 16 feet wide,” Fitzsimons shared.