Bern could only grow lengthwise.

01 A tongue of land in a meander

The city was founded in 1191 on a bend of the Aare. The river wraps the tongue of land to the north, east and south, leaving a single land approach to the west. That hill rises several tens of metres above the water: to hold up the terrace beside the cathedral, a wall of sandstone blocks about 31.5 metres high had to be built.

Walk the old town with that in mind and the plan becomes readable in five minutes. There is no grid and no radial centre: there are three or four very long parallel streets running west to east, with short cross lanes between them. You always advance in the same direction because there is no other direction to advance in.

02 12 May 1405

On 12 May 1405 a fire broke out in what is now Brunngasse. A strong north wind carried it across the whole city, which was built of wood. The fire took what was there, and the rebuilding used a different material: sandstone from the region's own bedrock, a grey-green stone.

That is the reason for the uniformity you notice on arrival. In many European cities the historic centre is a collage of periods and materials; here it is a compact block of one colour, because it was rebuilt almost entirely within a short span and with the stone that was to hand. What did not change was the plan: the rebuilding followed the earlier layout, because the ground was still the same.

03 Six kilometres under cover

And here comes the most visible consequence of all the above. In a city that can only grow lengthwise, the ground floor of every building is worth far more than in a city that spreads: it is the only possible commercial surface and it is all lined up along a few streets. The solution was to extend the building out over the pavement, resting it on pillars.

The result is the Lauben: some six kilometres of continuous arcades running along Spittalgasse, Marktgasse, Kramgasse and Gerechtigkeitsgasse, among the longest covered promenades in Europe. It is not decoration or whim: it is the way to double a street's usable frontage without widening it.

It has a practical effect you appreciate straight away: in a city with 1,025 millimetres of rain a year over 123 days, you can cross the historic centre end to end without getting wet. And it has a visual effect: the street does not read as an open space between two rows of houses, but as a corridor with a gallery down each side.

04 What was protected in 1983

UNESCO inscribed the old town in 1983, and the argument was not the beauty of this or that building: it was the ensemble as an example of clear, coherent urban planning, kept without substantial alteration since the twelfth to fifteenth centuries. What is protected is, once again, the plan and not the pieces.

That distinction is worth grasping because it changes the visit. There is no list of monuments to tick off here; there is a system you read on foot: the parallel streets, the continuous arcades, the fountains standing in the roadway, the vaulted cellars beneath the ground. None of those things is seen by going inside somewhere: they are seen by walking.

05 What Bern leaves you

What stays with you is a city that is extraordinarily easy to understand, and that is rarer than it sounds. In two hours of walking you have grasped why it is where it is, why it has the shape it has and why it is the colour it is. Very few European historic centres let themselves be explained in full by three facts: a meander, a fire and a nearby quarry.

And the physical sensation of the arcades stays, which is what you actually remember: walking under cover, in half-light, with the street lit on one side and the shop windows on the other, for hundreds of metres at a stretch. It is a way of being outdoors without quite being exposed, and it resembles nothing else on offer in other cities.