Not much is known concerning Cumberford's location in regards to Daventry, if it lies outside or is part of the kingdom. But it is at least treated as its own unique location to have its own regional name.
Cumberford is located near the kingdom of Daventry, along with the kingdom of Lycathia and the former kingdom of Sideria. It may be one of the minor western and southern kingdoms that share the continent along with Daventry and Serenia, where the people of Daventry fled during the destruction of the Three-headed Dragon. These are the many kingdoms the continent is divided into, most of them small and only a day or three's journey by foot in extent. These fiefdoms sometimes war or battle with each other for reasons their rulers care about, and shrink or grow depending on their martial fortunes. They seemingly come and go with the seasons, and few are remembered after their passing.
However, it is not clear where the nations are relation to each other, if they are close to each other, or near separate borders of Daventry.
The Cumberford men, and the young men at Duke Faust's court are said to likely be entranced by Rosella's beauty and her assets, much as she entranced the men of Daventry.
Behind the scenesEdit
The exact location of Cumberford is not known, but it is known that it lies about a week or less from Castle Daventry (or at least Graham and Valanice were staying there for more than a few days). The only other directional clue may come from the fact that upon the return trip, Valanice and Graham passed through the village south of the castle. While there are several roads, the south road, the north-south road, the central road, the east road that all converge on the village. For king and queen to pass through it to reach the castle without going out of their way, it would make more sense for them to have come into village from the southern areas of the kingdom. As the village lies roughly southeast of the castle. It is less likely to lie to the east, as that is a wild and unctrolled region between the Old Woods and Glass Mountains, so that largely leaves the South and Southeast as more likely locations for the land.
It is not clear if Duchy Cumberford is a sovereign state, or perhaps one of the lesser regions of Daventry under the rule of its Dukes and Earls. But it at least appears to be outside of the Kingdom of Daventry proper (or least outside of a possible province of Daventry) as Rosella compares the men of Daventry to the men of Cumberford at one point, though the book seems to only refer to Cumberford mainly in regards being outside of Castle Daventry, not necessarily the kingdom (and most scenes only mention the King and Queen are to return to the castle in a couple of weeks).
If sovereign nation there could be a few possibilities. The King's Quest Companion mentions the western and southern kingdoms being the only other kingdoms of note on the continent (although not nearly as important Daventry or Serenia), and the north and east of the kingdom is bordered by the Great Mountains (and at the most more or less unaffiliated frontier land leading into the mountains). At various times the only other kingdom of note in that direction is Serenia beyond the mountains (but during a period of time a huge section of that kingdom went missing due to the actions of Harlin).
Between the 'western kingdoms', and Daventry are the western woods where Morowyn lives (they lie within a day's travel from the castle), and the Twin Pillars of the Moon apparently make up the western edge of the kingdom.
If to the south, within the 'southern kingdoms', it would have to lie somewhere between the Plains of Jed (and possibly Zakizga) and Daventry. The plains of Jed lie about four weeks south of Daventry. It should be noted that boundaries of Daventry's rule at least at the time of See No Weevil does appear to stretch to a small region down near the southern sea (but not counting either Zakizga and the Plains of Jed). Other known regions to the south along the 'southern road' through Daventry and passing between sections of the Old Woods include a few farmlands, and the marsh where Sinofas lives (about three days away from Castle Daventry), which may be part of the Kingdom of Daventry (or at least implied to be).
It is less likely to be in the eastern lands beyond the Old Woods as that area appears to be non-affiliated frontier/wilderness (where only a few brave farmers live) at the foot of the Glass Mountains/Great Mountains and the sea.
It is less likely to lie to the north of Daventry. The shortest sections between the north and south borders (assuming the northern border is near the foothills of the mountains) lies within about a day or two journey (at least during the time of See No Weevil). Of note sections of the northern border lie about a days journey from the castle.The Swamp borders Daventry's northeastern corner, and lies outside of Daventry proper. The Vale of Willows (which lies somewhere between the Spasky Hills and Westering Mountains) lies about three weeks to the north of Daventry, apparently high up in the mountains.