Today the inhabitants of Paris begin their annual holiday exodus, happily surrendering the city to foreign tourists. It was different 800 years ago, when King Philippe Auguste was off to the Crusades: he was so concerned that the likes of Richard the Lionheart might visit the capital during his absence that he ordered the construction of a stone wall nearly five miles long, close to 30ft high and 10ft thick at its base, with watch towers every 70 yards or so.
Modern-day invaders who want to find the remnants of these impressive fortifications need to know where to look. In all but a handful of places, the only clues might be in a street name (if it includes the word fossé, or ditch, it refers to the dry moat that was outside the wall the odd alignment of a building, or a stretch of stone glimpsed through the gate of a courtyard. Tracing the line of the wall takes you behind a fire station and even into an underground car park, lending a new perspective on some well-trodden parts of Paris.
I followed the Left Bank section, which skirts the Latin Quarter, but first crossed to the other side of the Seine, where the largest fragment of the old ramparts is found in a recreation ground next to Rue des Jardins de St-Paul. With an unbroken stretch of wall between the remains of two towers, you can visualise what the whole was like.
This helped when I returned south of the river to Rue de Cardinal Lemoine, where there is a plaque near the Paradis Latin cabaret which describes the scene eight centuries ago. Cross the road and you discover that, above the nightclub's tacky façade, the upper storeys of the building are at an oblique angle. They follow the old line of the wall.
Round the corner, 2 bis Rue des Ecoles is strangely narrow – the sandwich bar, and the storeys above it, fill the space the wall used to occupy. But to see actual stones, go down the side of the fire station at 50 Cardinal Lemoine. At the back is a section of 13th-century wall with courses that follow the slope of the hill, defying a spirit level. One theory is that the builders were trying to keep some of the materials for themselves.
If these remains take some sleuthing, there is no mistaking the cross-section of wall looming over Rue Clovis, a little higher up Cardinal Lemoine to the right. Here you can see the base in all its thickness, the rubble that filled the cavity higher up and the crenellations that topped the original construction before it was built even higher in later centuries. Other sightings of the wall nearby, at 7 Rue Clovis, 62 Cardinal Lemoine and 16 Rue Thouin, depend on gates being open for commercial or government business, so it is better to do this walk on a weekday.
The ancient remains become more elusive at the top of the slope, but in Rue Soufflot, Number Five follows the line of the wall rather than the street, which is centuries younger. From here you can walk down Rue Monsieur le Prince (formerly Rue des Fossés Monsieur le Prince, which tells you that the wall used to run here), though there is little to see; you might prefer to visit the nearby medieval treasure house that is the Cluny museum.
But it would be a tragedy to miss two of the most remarkable survivals of Philippe Auguste's fortifications. Head for the Cour du Commerce St André, a cobbled lane just off Boulevard St Germain that is itself a historic relic. Opposite the famed Café Procope, the entrance to a courtyard creates a gap in the line of shops. Look through the window to the right, and in the middle of the shop is the base of a 13th-century tower, its scrubbed golden stonework looking brand new.
A little further on is the last token of the late king's work on this side of the Seine, and the strangest of all. Turn off Rue Dauphine and walk through the charming Passage Dauphine – the dog-leg in the middle echoes a corner of the ancient wall. Note the language school on the left.
As soon as you emerge from the passage on to Rue Mazarine, there is an underground garage to your left, at No 27, and from the street you can see old stone at the bottom of the ramp. Take the pedestrian entrance beside the ramp – the attendants are used to history hunters – and you can marvel at the sight of parking bays closed off by a section of wall so deep that the base is another level down.
And the language school above? You can ask to see the tower that flanks a lecture hall. The hall is named after Philippe Auguste, which seems only fitting.
For more information, see philippe-auguste.com/uk