So (according to the concept art book) as the Fellowship travels deeper into Middle Earth, the places they pass through become inspired by progressively older periods of history. The farther along you are in the story, the more ancient the design influences
We begin in The Shire: which feels so familiar because, with its tea-kettles and cozy fireplaces, it’s inspired by the relatively recent era of rural England in the 1800s
But when we leave Hobbiton, we also leave that familiar 1800s-England aesthetic behind and start going farther back in time.
Bree is based on late 1600s English architecture
Rohan is even farther back, based on old anglo-saxon era architecture (400s-700s? ce)
Gondor is way back, and no longer the familiar English or Anglo-Saxon: its design comes from classical Greek and Roman architecture
And far far FAR back is Mordor. It’s a land of tents and huts: prehistoric, primitive, primeval. Cavemen times
And the heart of Mordor is a barren lifeless hellscape of volcanic rock…like a relic from the ages when the world was still being formed, and life didn’t yet exist
And then they finally reach Mount Doom, which one artist described as
“where the ring was made, which represents, in a sense, the moment of creation itself”
Tolkien was always very precise with the symbolism in his stories; he planned out everything from the seasons, to the levels of technology, to the terrain and geology encountered along the journey in LotR.
The visual timeline in the films mirrors both the in-universe chronology of Arda, as well as the “felt” chronology of the stories– with the hobbits representing the modern everyman journeying back through more “heroic” ages, encountering relics from a distantly-remembered past. I think Tolkien would have appreciated that the concept artists took such care with this effect (whatever his other quibbles may have been in terms of direction).