The temple is dodecahedron - it is one of platonic solids composed of 12 regular pentagonal faces. Aristotle mentions it as a building block of cosmos (Aether). Pentagon is obviously an important symbol in the series. Its is interesting that Mother and Father raise 6 children, then FIVE die and only Campion remains, but later Mother brings another FIVE. Dodecahedron is comprised of 12 pentagonal faces and Campion is 12 years old... There are some interesting symbolic parallels.Maybe they are just coincidences, but nevertheless it is intriguing.
Regarding that mysterious figure living in the serpent's head, its still hard for me to guess who that might be, but the whole scene under the snake's head reminded me of
https://en.wikipedia.org/wiki/Protevangelium At leas on symbolic level.