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”