As Ambrose already mentions in the books, Place Denfert-Rochereau was once called Place d’Enfer – Place of Hell. I’m yet to find out why exactly. All I know is that Place d’Enfer was the area just outside of the former city gate called Barrier d’Enfer that marked also the tax limit of Paris. The two buildings that once stood left and right of the gate are still visible at Place Denfert-Rochereau. In fact, the entrance to the Catacombes de Paris is right next to one of them.
Why the name change? Pierre Denfert-Rochereau was a French commander who fought in the Franco-Prussian War of 1870/71. Supposedly, he was so brave that he got nicknamed the Lion. Yet, the French lost this war and the moral was pretty low. In an attempt to raise it again, the government renamed places after French heroes of the time. That d’Enfer and Denfert sound almost the same came in handy as well, since that way no one could really complain about discarding of tradition by renaming a place. And yes, the lion in the middle of the place praising the French defense that’s supposed to be Denfert-Rochereau.
As I said, the entrance to the underground ossuary called Catacombes de Paris is located just south of the former city gate. There is some confusion about what exactly is meant when one talks about the Catacombes. Underneath Paris exits a whole network of tunnels and caves that are the remains of Paris’ stone mines. E.g. Rue du Bac is named after the ferry that transported the stone blocks from these mines to the construction site of the Palais Tuileries on the other river bank. It was originally called grand chemin du bac (great way to the ferry). The whole network underneath Paris, which is the real reason why where aren’t that many tall buildings in Paris (no room for large foundations) is called les carriers de Paris. Only a small renovated section was filled with the bones of ca. six million people. This ossuary was opened in 1788 and is some kind of tourist attraction since the 19th century. But a lot of times people talking about the Catacombes really mean all the tunnels and caves, not just the ossuary.
Oh yeah, the Catacombes are of course the place where Lucien takes Charles body to lure the Revenants out of the house in order to have an easy time to destroy Vincent’s body.