Riga looks like one enormous building because it was built almost all at once.

01 A city with one date

The effect is disorienting in a way that takes a while to name. You walk down a street in the centre and every façade is elaborate — masks, garlands, women's faces four floors up, peacocks, tiled panels, wrought-iron balconies — and yet nothing feels like a monument. There is no single building to photograph, because the building is the street. Riga does not have Art Nouveau landmarks. Riga has Art Nouveau as a background condition.

02 The port that tripled

The money came from the sea. Riga was one of the great ports of the Russian Empire, and in the decades before the First World War it grew at a rate that is hard to picture now: the population rose 88 per cent between 1897 and 1913, reaching 530,000 people, which made it the fifth-largest city in the empire. Timber, flax and grain went out; industry and workers came in.

A city that adds a quarter of a million people in fifteen years has to house them, and it has to do it in apartment buildings, quickly, for rent. That is the crucial point about Riga's Art Nouveau and the one most visitors miss: this is not a district of villas for the rich. It is speculative rental housing, built for profit, decorated to attract tenants. The extravagance on the façades is a sales technique.

03 The walls came down first

There was also room to do it, which is the other half of the explanation. Riga's medieval fortifications were demolished between 1857 and 1863, and the belt they left behind — a ring of green with a canal running through it — separated the old town from everything that came next. Beyond that ring the new city was laid out on a strict grid, with regulated building heights.

That regulation is why the streets read as single objects. Every block runs to the same height, so the cornice line is continuous for hundreds of metres; the plot widths are similar, so the window rhythm repeats; and the ornament, which is the only free variable, goes wild inside a very disciplined frame. It is the opposite arrangement to a picturesque old town, and it is much rarer: a whole quarter designed as one piece and then decorated as thousands.

04 Four ways of doing the same thing

Once you have the date, the next useful thing is to learn to tell the four kinds apart, because they are not the same argument and they are all on the same street. The first is the decorative, eclectic version: masks, mythological figures, symmetrical compositions borrowed from history. Mikhail Eisenstein is the name attached to the most exuberant of these, and Alberta iela is where they are concentrated.

The second is what is usually called perpendicular, and it is about a third of the total: vertical bays running the full height of the façade, far less ornament, a rational look that already points toward the nineteen-twenties. The third is National Romanticism, another quarter to a third of the buildings: steeper roofs, heavier natural materials, motifs taken from folk art, associated with architects like Konstantīns Pēkšēns and Eižens Laube. The fourth and rarest is neoclassical, and it is mostly banks — monumental, sober, expensive-looking.

There is a fact worth carrying with you while you look at all this: Latvian architects designed nearly 40 per cent of the buildings put up in the city in the early twentieth century, at a time when Latvians were a minority in their own capital. Pēkšēns alone was responsible for a very large share of the centre, and the flat where he lived, on Alberta iela, has been the Riga Art Nouveau Museum since 2009 — the only place where you can see a complete interior of the period rather than a façade.

05 The city that was there before

The other half of Riga's UNESCO listing — the historic centre was inscribed in 1997 — is the part almost nobody photographs: nineteenth-century wooden architecture. Before the boom, this was a city largely built of timber, in low buildings with plank cladding, shutters and carved details, and a surprising number of them are still standing in the districts just outside the centre.

Seeing the two side by side is what makes the story land. A wooden house of 1813 and an apartment block of 1910 are separated by a century, but in Riga they are also separated by a change of scale so violent that the second could swallow twenty of the first. The stone city did not evolve out of the timber one. It was dropped on top of it.

06 How to walk it

The practical advice is to stop treating the centre as a checklist. Alberta iela is the famous street and worth the twenty minutes, but it produces a distorted idea of the city, because it is the most theatrical version of one of the four styles. Walk Elizabetes iela and Strēlnieku iela for the same period at a calmer volume; then pick any residential street in the quarter at random and notice that it is still happening there, without an audience.

And look up more than you would elsewhere. All of the argument is above the ground floor: the shops change every decade, the plasterwork does not. Riga is a city where a hundred and ten years of ordinary life have gone on underneath one continuous decorated ceiling — put up in a hurry, by developers, in the last decade before everything changed.