Baku is three cities stacked on top of each other and a single well.

01 The stone city inside the wall

The first layer is Icherisheher, the walled inner city, inscribed on the World Heritage List in 2000. It keeps a good part of its twelfth-century defensive walls and stands on a site inhabited since the Palaeolithic, with successive traces of everyone who passed through.

The piece that sums it up is the Maiden Tower: an eight-storey cylinder, 29.5 metres high and 16.5 across, raised in the twelfth century over earlier structures going back to the seventh and sixth centuries BC. At the highest point of the enclosure stands the Shirvanshahs' Palace, twelfth to fifteenth century. All of that fits inside a perimeter you can walk in an hour.

02 1900: half the world's oil

And then comes the figure that reorders everything. By around 1900, this country was producing half the world's oil. Not a substantial share: half. In 1879 the Nobel brothers had founded their company here, and a year earlier they had built the world's first oil tanker to move the product across the Caspian.

It is worth pausing on what that means for a city. In two decades a provincial Caspian port became the centre of a global industry, with Swedish, French and Russian capital, and with a class of newly rich people who needed somewhere to live and something to prove. That combination —sudden money, a great deal of it, and the urge to display it— always produces the same thing: architecture.

03 The ring of mansions

The second layer is the ring around the wall: whole blocks of pale stone buildings from the late nineteenth and early twentieth centuries, with façades from the European repertoire —cornices, balustrades, arched windows, columns— commissioned by those who grew rich on the wells. It is a belle époque built five thousand kilometres from Paris and paid for in crude.

It is worth walking that ring with no particular objective, looking up. Many of those buildings have changed use three or four times since —homes, offices, museums— and the state of repair is uneven, but the scale and ambition of the façades still say where the money came from.

04 The third layer, in glass

The third layer is recent and visible from anywhere in the city: a group of curving bluish-glass towers on the hill commanding the bay. They answer to the same mechanism as the mansions, a century on: extraordinary income from a resource underground turned into architecture visible from far away.

What is interesting is not each layer on its own —there are better walled towns, better preserved belle époque and more ambitious skyscrapers— but the distance between them. From a rooftop in the old city you can see all three at once, and the walk from the first to the third takes twenty minutes.

05 What Baku leaves you

What stays with you is a lesson in cause and effect rare in a city. Usually you have to reconstruct why a place is the way it is: here the explanation is in plain sight, and there is only one. The century changes, the style changes, the architects change; where the money comes from does not.

And a question stays that the city poses without saying it. All three layers belong to boom periods, and all three were built while the resource was still delivering. Walking from the wall to the towers is walking a hundred and fifty years of successive booms, and realising that none of the three layers was built with the next one in mind.