The park is a good place to contemplate the Enceinte de Thiers. The photograph below (taken at the Porte de Versailles) shows how these fortifications functioned. Deep trenches and walls up to 10 metres high surrounded the whole city, with gateways (the Portes that still exist today) letting people in and out of Paris. Beyond this area, a buffer of another 250 metres where construction was forbidden, and which came to be known as the 'zone'. Designed and built in the 1840s, the defences proved to be a spectacular failure, notably during the Prussian invasion in 1870. They were eventually destroyed and removed in the 1920s, but in a certain sense they are still very much in place today.
It is very rare to find traces of the fortifications today, but some of the stones are still in place in the Parc Kellermann. Beyond these physical remnants though, the park itself is very typical of the kind of equipment that sprang up in the zone and in place of the old walls and trenches.
Where previously there had been a paranoid emptiness, today there are parks, football pitches, swimming pools and red-brick social housing. The view from the top today is green, across a buffer of sport and leisure, but the psychological circle around the city still exists.
The edge of city is still solid and very clearly defined. From these walls in the Parc Kellermann today we still look out beyond Paris, across a football pitch, the periphèrique motorway, and out to the suburbs. Invaders are no longer to arrive, but the city still looks outwards with suspicion.
These walls were already a sign of the city's desire to dominate and control its neighbours when built. Many of the surrounding villages were annexed by Paris in 1860, but these walls had already extended the reach of the city out beyond them anyway, cutting several of them in half in the process. The villages had become an extra cushion of defence for the centre of Paris.
But what purpose does this ring around the city serve today? When the fortifications were pulled down, the planners simply wanted to fill the space in the cheapest manner possible, and parks and stadiums were judged an ideal solution, particularly in a city that lacked such facilities. By preserving the line of these defences though, the planners also ensured that Paris would still be cut off from its neighbours nearly 100 years afterwards.