
James Duncan. "The Quebec Tandem Club, Champ De Mars, Montreal" (detail), ca 1840, Royal Ontario Museum, 953.186.1.
Originally, a “field of Mars” was a space set aside for military drills and parades. The name dates back to Antiquity: Mars was the Roman god of war.
Montréal’s has existed since 1740. After the city’s fortifications were demolished, the esplanade was expanded in 1812, becoming a broad terrace behind the institutional buildings on Notre-Dame Street.

John Elliott Woolford, "On the Parade at Montreal", ca 1819, Library and Archives Canada, 2887663.
After the fortifications were demolished, the Champ-de-Mars esplanade was redesigned and lined with tall poplars, which quickly became its signature feature. They are easy to spot in 19th-century images.
Another striking detail: the staircase linking Notre-Dame Street to the large esplanade.