In the Roman times this was the main entrance to the city of Nemausus. The bigger archs are for vehicules, smaller ones for pedestrians. As you can imagine already from this picture, the city of Nemausus was about half a meter lower in the ground than today’s Nîmes. The next picture gives you another angle:
The Porte d’August, also known as Porte d’Arles, was named after Augustus, who introduced the city walls to Nemausus in 15 BC. In the 14th century, it was incorporated in the fortress, which again was destroyed during the Revolution. To prevent any further damage, the Roman Gate was listed in the very first list of Monument Historique of 1840.