Much of Medieval America is warlike with towns raided, and maybe wiped off the map. In one corner, we see vast, despotic empires, in others marauding tribes, and in the bulk of the continent, a patchwork of feudal states. However, in the Northwest and Northeast coast, we see independent city states which are separate, but often share cultures, although they're very cosmopolitan overall. These cities are not ruled by commanders in chiefs, so as established town bylaws and a ruling class descended from merchants.
In most of Inland America, we see warriors fighting bitterly for territory. However, Warlords and War Chiefs are not the only ones to call the shots. There's also the growing merchant class; Those who deal in exotic goods, and make sure the nobles in one country have access to materials and delights they cannot procure in their own borders, or closely conquerable kingdoms. They tend to congregate in cities with large populations, because not only do the rustic farmers and shepherds provide a poor customer base, but a smaller labor pool means less artisans to specialize in goods to buy and sell. In most of America's kingdoms, the merchants are a valuable part of society, and can live the good life with influence and relative freedom, but ambition has a ceiling, and the governors can't be trusted. Thus, many might live a traveling life, or gather in the city states of the Northern coasts.
The Merchants are particularly suited here. Here, various bays, inlets, islands and sounds are ideal places to set up ports. The North and South have considerably different things to offer, and the Northeast and Northwest are perfect hubs to connect trade. It's also the result of being the location of former bustling metropolitan areas. Here, the municipalities are built to human scale. The interior of the continent is much newer, and the populace distributed after the age of mass transit like trains and planes. So when places like the Midwest and the deep south fell, communities could be taken over by warlords with little to answer to. However, in Cascadia and the Northeast, major cities and their suburbs are were tightly bound together. This means not everybody can be a farmer, and there's a larger pool to draw employment from.
It also helps that these places can headquarter the local religions. In the Northeast in particular, we see many headquarters for the Non-Denominational Church in proportion to populace. As a result, the Church brings it Republican bureaucracy to the populace. More public works and social services are implemented, and the populace is largely more educated. Many holy buildings can keep these places somewhat free from devastating attacks; While the leaders of city states may conquer each other, it's widely frowned upon to slaughter the local population or clergy. Because of this, many people feel relatively safer in the city states, leading to more people per capita, and denizens not being full-time fighters.
Many of these Mercantile City States are located in what were once known in Industrial America as "blue states". This no mere coincidence. The values of Red-Staters and Blue-Staters, North and South, Democrat and Republican, were unquestionably tied to the rural vs. urban dichotomy. Blue states had higher population density, and more cities. Therefore, they valued a strong bureaucracy, catering to a varied population, and a relationship between other countries. Red states tended to have their communities spread wide apart, and valued self-sufficiency, defense, and loyalty for local traditions.
This is not to say the Mercantile Republics are averse to war. On the contrary, many merchants will constantly war to expand their realms. It's simply that the leaders are not as trained in the martial arts as the feudal warlords. They delegate duties to career soldiers, and mostly weight in with tactical maneuvers. The city states' relationships with each other are different than the kingdoms and tribes found elsewhere on the continent. When nations in the Heartland, Deserts and plains war against each other, it's generally with the purpose of wiping the enemy out. However, the mercantile city states will usually share among themselves a similar culture like in New England, the Beltway and Vancouver Sound, and wars among themselves are something of a fierce rivalry.