The Northern Alliance was a term first used during the Coming of the Order to refer to the villages of Kimrin, Port, and Near Port (the latter of which was founded in 897, at the very start of the Coming, by settlers from Kimrin, Port, Ship, First Village, and Monab). At that time, the Order began introducing some of its plans for modernizing The Land, with the Northern Alliance as a sort of test run. In 901, they also began raising armies and navies in those villages, in anticipation of dissent against the Coming from elsewhere in the world. (Of course, over the years the Order had been slowly guiding every village toward desiring its own goals, even if they didn't realize it at the time; so people from other villages would eventually join the armies and navies, as well.)
After the Second Order was established, the term "Northern Alliance" ceased to hold any real meaning, as all the villages of the world were united and equal in every sense, and the three northern villages aren't in any official sense more strongly aligned with each other than with the rest of the world. However, the name has stuck, for the simple reason that those villages are, as the name suggests, much farther north than any other villages on the Land, and geographically closer to one another than they are to the other villages. This, naturally, makes the citizens of these villages more likely to travel within the Northern Alliance than to the southern villages, though of course there is contact between the N.A. and the rest of the world, in the form of trade, politics, and culture. It should also be pointed out that citizens of the N.A. experience much more drastic changes in seasonal weather than people in any other villages. This leads to them often complaining about the cold in Winter, and therefore makes it popular for those N.A. citizens who can afford it to vacation in the Land's warmer villages. On the other hand, some people from the southern villages like to travel to the N.A. precisely so that they can experience Winter weather, which they'd otherwise only heard of in stories from other worlds such as Earth.