"It was a furious battle, for the savages had warslings to help them. Karlsevni and Snorri watched them lift up a pole with a huge knob on the end, black in color, and about the size of a sheep's belly, which flew up on land over the heads of the men, and made a frightening noise when it fell."​

"Ballista, or Demon's Head. — Algonkin tradition affirms that in ancient times, during the fierce wars which the Indians carried on, they constructed a very formidable instrument of attack, by sewing up a large round boulder in a new skin. To this a long handle was tied. When the skin dried, it became very tight around the stone, and, after being painted with devices, assumed the appearance and character of a solid globe upon a pole. This formidable instrument was borne by several warriors, who acted as ballisters. Plunged upon a boat or canoe, it was capable of sinking it. Brought down among a group of men on a sudden, it produced consternation and death."​

In the Saga of Vinland , there is a mention of pretty large "warslings" used by the natives:In an 1884 book describing the culture and history of Native Americans (pg. 73):(Further historical discussion of this on AskHistorians . I got the idea from there.)Let's say that the stories of this device have some truth to them. (The Sagas were pretty untrustworthy, but an unfamiliar swinging weapon doesn't seem like something a Norse or Algonquin storyteller would invent--it's not a mythological beast or anything).What if the natives of northeastern North America continued to develop this device, possibly even adding a wheeled cart to it? (Vikings introducing the wheel and cart.)Obviously, this ballista/sling/catapult would not compare to cannons. But as it would provide a considerable advantage in warfare, its use might expand outward. Native cities in the East Coast weren't very dense, but it still might be necessary to build walls around larger ones. If the ballista spreads to the Mississippian cultures, who had much larger cities, then they might start building sophisticated fortifications. If it makes it to the densely populated Mesoamerica, polities with a great deal of resources for war, siege technology might start evolving as quickly as in the Middle Ages.After all there were a full 500 years between the settlement of Vinland and the Spanish arrival in Mexico.What if the conquistadors encountered a Mesoamerica and Gulf Coast where most large cities had walls around them, and experience with several types of siege engines? Maybe native polities would more quickly adopt gunpowder and cannons as well, since they'd already have some sort of artillery.