Paris is surrounded by a ring road. From this road, you have multiple entry points to get into the city, called "portes". Each porte is named for the place where the road will take you. It doesn't make a lot of sense today, but before there were numbered routes and highways, you knew that if you left the city at the Porte du Versailles, you would end up in Versailles.
Today's Paris is much larger than the Paris of yesteryear. The organic growth of the city started at Ile le cite, where Notre Dame is, and gradually spread to both banks of the Seine before taking over more and more land, until suburbs like Montmartre were enveloped. Evidence of original city walls and gates can be found within the city limits. While the walls protected, the gates were toll roads to get in and out of the city, and for traders to import and get taxed on goods.
A few of the Ancient gates of Paris still stand, two of them triumphal arches side by side not far from Republique. They are lovely. I did trek all the way to them with my camera, only to find out that the battery was drained when I got there (thanks, cold) and went back the next non-rainy day I could.
The Porte Saint-Martin was built in 1674 to replace the original medieval gate:
A couple blocks away is the slightly older 1672 Porte Saint-Denis:
See all those pigeons?
Ugh, yes. Paris has a rather strange pigeon birth control scheme. They build these big pigeon-houses for them and scatter them about the city. Once the pigeons had laid eggs a couple times and feel safe there, someone goes in and shakes all the eggs in the spring, killing the fetal pigeon but making sure that the adult pigeons will continue to incubate the egg until it disintegrates, thereby leaving them unwilling to breed. I'm not sure if it actually works or not, but you will know if you are near a pigeon safe house as the ground is thick with grazing herds of them. It's like the Serengeti, but absolutely disgusting.
So yes, there is a pigeon house near Porte Saint-Denis.
Sorry about the lack of color or contrast- I was dealing with another flat gray day.