In this chapter, Zinn is not really interested in telling the story of what made some settlements “key” for the English. He is interested in talking about how the English (and other Europeans) treated the Native Americans. Zinn talks about the English settlements in this context.
One “key” settlement that he mentions is Jamestown. This settlement, in Virginia, was the first permanent English settlement in North America. Zinn mentions it because its presence led to the abuse of the Native Americans. Zinn says:
Not able to enslave the Indians, and not able to live with them, the English decided to exterminate them.
Zinn tells a similar story about the other key settlement in English North America. This was the famous Puritan/Pilgrim settlement in Massachusetts. This was the settlement that began with the Pilgrims coming over in the Mayflower. Again, Zinn argues that the English committed atrocities against the Native Americans. He says
The English developed a tactic of warfare used earlier by Cortes and later, in the twentieth century, even more systematically: deliberate attacks on noncombatants for the purpose of terrorizing the enemy.
So, Zinn does mention two English settlements, Jamestown and Massachusetts Bay, that are “key” in our usual histories. However, he talks about them mainly to show that, in his mind, the English treated the Native Americans in immoral ways.