Alien : Ridley Scott's Early Egg silo exterior adapted from "National Park" in Giger's Necronomicon

leading from

Alien: The Egg silo
Ridleygrams,

Prometheus: Pyramid in The Prequel
and
HR Giger's "National Park"

a) The Egg silo based on the Castle Harkonnen

Ridley's Egg Silo sketch

Giger's Dune II Castle Harkonnen

Giger's Dune I Entrance to the Castle Harkonnen.
(See the spikes on either side of the entrance pathway 
for the egg silo leading up to the mouth in the next image)

See: Designing castle Harkonnen

b) Entrance of the egg silo based on a head from HR Giger's Painting "National Park"

Ridley's sketch of head on Egg Silo

A skull from Giger's painting National Park from 1975.

clearer version of the image

Giger's painting "National Park " from 1975.