They build on the crests and farm in the hollows.

01 Three storeys, not seven hills

The first unit is a barrier of inselbergs: isolated rock hills standing out of the ground like islands. To the north-west are Mount Mbam Minkom, at 1,295 metres, and Nkolodom, at 1,221; to the south-west, Mount Eloundem, at 1,159. The second unit is the set of rises and plateaus at 600 to 700 metres where almost everything built stands. The third is the valleys.

02 The élobis

The valleys have their own name in the local language: élobis. They are waterlogged bottoms, run through by a multitude of small watercourses — the Mfoundi, the Ékozoa, the Biyeme, the Mefou — and for a long time they were considered useless ground to build on.

That division is what stops Yaoundé from resembling any other capital of its size. From above, what is built appears as islands on a continuous green ground, and that green is not parkland: it is market garden, maize plot and banana grove inside the municipal boundary. City and country are not separated by a line, but by forty metres of height.

03 Twelve square kilometres

The figure that best measures what has happened here is one of area, not population. In 1961 the city covered twelve square kilometres. By the year 2000, three hundred and four. Twenty-five times as much ground in under forty years.

At that rate, the hollows stopped being useless ground. Geographers who have studied the city describe how self-built housing has taken over the flood-prone parts, precisely because those were what remained free. And that brings the predictable consequence: until recently the city flooded severely between fifteen and twenty times a year, sometimes affecting a hundred thousand people at once.

In 2010 a sanitation master plan was launched. Four years later the frequency of flooding had fallen from fifteen to three times a year, and cases of water-borne disease had dropped by almost half. It is a work invisible to the visitor and it is the most important one in the city.

04 The edge of the forest

One fact orders all the rest, and it is the oldest. The original post was established in February 1888 at a very specific point: the northern limit of the forest, and midway between the area's two great rivers, the Nyong and the Sanaga. The city is neither in the rainforest nor outside it. It is exactly on the edge.

You notice it leaving in any direction. To the south the vegetation closes in and does not open again for hundreds of kilometres; to the north it thins. And you notice it inside, too: the élobis are, at bottom, shreds of that forest brought into the city and converted into gardens.

The site sits at about 750 metres and 250 kilometres from the coast of the Gulf of Biafra. The altitude matters: it takes enough off the equatorial heat for the annual mean to settle at 24.4 degrees, in a place less than four degrees from the equator.

One practical rule serves the traveller. If you want to understand the city in an afternoon, do not go down into the centre: go up. From any of the high points the whole mechanism is visible — what is built on the crests, the green in the hollows, and the inselbergs behind — and from there the rest of Yaoundé reads itself.