Haraldsborg is a city built at the site of OTL Manhattan, named after Prince Harald of Denmark, official discoverer of Atlantis. In 1387, on his second journey, he came as far as to the island that was named Prince-Harald-Island in his honor and founded a settlement that would become Haraldsborg.
When the Black Death hit Atlantis in 1435, the Danes had to give up most of their settlements, but Haraldsborg survived. 1438, when Denmark and the Netherlands divided the New World between them, Haraldsborg stayed Danish.
1509-12: After some clashes with Danish colonists in Atlantis, the Quadruple Monarchy declared the anti-Danish War against Denmark. The Dutch allied with them. In the peace of Hamburg, Denmark lost its lands in OTL Canada to the Quadruple Monarchy and Anderland to the Netherlands. Only the city of Haraldsborg and Prince-Haralds-Land (Newfoundland, as a base for fishermen) stayed in their hands. And even worse, 1544 in Haraldsborg chaos ensued after the mother country fell into civil war. Now Poland sent soldiers to the city, occupying it. Haraldsborg became the portal for Polish and Prussian immigrants to (Northern) Atlantis.
When king Alasdair IV was elected Polish king, he controlled Haraldsborg again, helping to calm down the relations of Danes and Poles in the city. His reign stayed a short episode, however; and in the English-Polish War 1613/14, Poland lost OTL Trinidad and Haraldsborg to England, making the city a part of New England.
1637, however, the Dissenter's Revolt happened in New England. Denmark-Braunschweig, supported by France, used the situation to expand there / take old lands back. Only Markland (OTL Canadian Maritimes and Quebec) could defend itself during this decades, ironically, thanks to its strong garrison. Denmark-Braunschweig got Haraldsborg back, plus became protector of the Commonwealth of New England (the dissenters preferred "the papist German king in Europe to the papist English governor in Markland"). German immigration now also went to New England, where many new settlements were founded. English loyalists emigrated to Markland. In 1642, the colonies of Denmark-Braunschweig (Martinsburg, Waldstätten, Nieuw-Nederland, Haraldsborg and Anderland) were united under a common administration.