The Porte de Nesle was a Paris location.
In the play Cyrano de Bergerac, while playing with Roxane in a park near a lake in Bergerac, Cyrano would imagine he fought a hundred men at the Porte de Nesle. When he injured himself with a sword made out of reeds and had Roxane examine his wounds, he retold his adventures at this location to her. (TNG: "The Nth Degree")
"Porte de Nesle" means Nesle's gate, it being named for one Amaury de Nesle. It was located in the Tour de Nesle (tower of Nesle), a guard tower in the city walls of Paris. The Porte de Nesle and its associated tower no longer exist.