The Barriere d'Enfer is a pair of neoclassical toll pavilions at the old southern entrance to Paris, designed by Claude-Nicolas Ledoux in the 1780s. They were part of the Wall of the Farmers-General, where goods entering the city were taxed before the wall was largely demolished.
The pavilions stand by Place Denfert-Rochereau, close to the entrance to the Paris Catacombs. They are a quick architectural stop, but a good one if you are interested in the old limits of the city.