An Arc de Triomphe is a Triumphal Arch, a concept dating back to Roman times. This particular arch was built by Napoléon I^er in the style of the Arch of Septimius Severus on the Forum Romanum.
In the same way at that Roman arch, it commemorates an army and, obviously, a triumph, namely that of the Grande Armée in the years prior to its construction which began in 1806.

But what is a carrousel? In this case, the term refers to a type of military dressage. The Place du Carrousel where the arch is located takes its name from the Grand Carrousel, which took place there on June 5-6, 1662, on the orders of Louis XIV to celebrate the birth of his firstborn son Louis (who despite being the Dauphin, did not become king after his father).

The arch was not yet part of a special axis, even though planning and construction of the Arc de Triomphe de l’Étoile began the same year. In fact, it was built right in front of the Tuileries Palace that closed off the Louvre on its western end and became its gate of honor.