In the context of the last post, I am granting no special dispensations for humans or any other form of humanoid; in game terms, anything that can cause a hit point of damage to another creature is a "monster." Therefore, humans are builders just as any other builder monster.
Moreover, I should like to take a moment and point out that a given monster might fit various monster types as described. Most humans are builders; but humans can easily be malevolent destructive wanderers and a given powerful human might be a very effective eradicator. The "type" of monster refers to the monster's behaviour and does not necessarily encapsulate every example of a given monster species.
Within the framework, we can describe a "civilization" as any builder monster that has effected a sufficient change to the local environment that the only other monster that can co-exist within the builder's sphere is necessarily verminous in nature. This does not mean that the vermin are necessarily minor in form. A vampire might live inside a civilization, in the catacombs under a city or in plain view, as in the Bram Stoker novel. Howeer, while in the wilderness a vampire might be a very efficient eradicator, in a civilized culture of thousands of builder monsters the vampire must act covertly, or else risk arousing the most powerful of the builder culture leaders to root it out and kill it. It must be clearly understood that along with reshaping the environment, builder monsters have also shaped their wisdom and capabilities, making builder monsters the most dangerous monsters in every world.
A civilized space may include areas of hinterland, but it should be specified that these areas are deliberately allowed to exist, because they provide forage for domesticated monsters and as hunting reserves, as well as potentially enjoyable places for excursion among the wealthiest and most powerful builder monsters. True wilderness is not a forest reserve, which has been tailored and picked over by wardens, and thus cleaned of any monsters except for those pesky vermin, which never truly disappear. True wilderness is back country that no one has as yet been able to clear out, or have not been motivated to clear out, which is the point of the rest of this post.
When gauging which parts of the reader's world are fit for civilization, it should be assumed that builder monsters have already chosen to occupy the best lands, without exception. If the technology of your world has had the opportunity to produce ocean-crossing vessels, and has existed for thousands of years, then at the very least there is a war going on to remove other monsters from good lands that have only been recently discovered by builder monsters. That is effectively what is going on in my world in 1650; all this wonderful, prime land has been discovered in the last 150 years and is undergoing a violent transfer of ownership, as builder monsters move en masse to the coastline along the western shore of the world's second largest ocean. This prime land is being renamed "America," despite the names formerly given to the lands by passive wanderers (or builder monsters with insufficient technology to withstand encroachment).
Wilderness, therefore, describes those places of the world that are largely second-rate in the eyes of builder monsters. This may include a well-watered plain, but it will be a small one with poor access to the outside world and most likely in a very cold or a very dry and unreliable climate. Therefore, when wandering through the wilderness, we shouldn't expect to find any large builder cultures ~ at best, we should find only primitive ones. Any large builder culture will proliferate in population and a desire for outside contact, which will result in the establishment of some kind of trade, followed by the importation of technology and thereafter a rapid civilizing of the wilderness in the manner I've described: that is, the wilderness will be fully rebuilt.
Such a place would be the jumping off point into the wilderness, the last vestige of civilization the players were leaving; it would not be a place the players discovered completely by chance just by wandering fifty or sixty miles after "leaving" civilization. Everyone in the civilization where the players left would know about the island of civilization the players were "discovering" already. Such a civilization would already be on the map.
Therefore, builder monsters in the wilderness would be extremely limited by the size of their environment. They might occupy an oasis, but it won't be a big, massively productive oasis. They might have settled into a valley, but it won't be a rich, wide, flat valley that might support thousands. More likely, it will be a somewhat chaotically arranged narrow valley that will support only scores, certainly no more than several hundred. Such a group of builder monsters, sufficiently separated from actual civilization, could be overlooked by previous explorers and may have only settled in the valley a generation or two before. Perhaps the valley was cleared out a hundred years ago by a group of civilized adventurers and has now been resettled by something else.
Any builder society on the wilderness level, as I've tried to describe, would not have fully gained control over their environment. They are co-existing with that environment, which means that they are at the mercy of the occasional passive wanderer or substantial group of vermin. A single destructive wanderer could eliminate the whole society. They would certainly not have the power to send out soldiers to end the power of an eradicator whose world begins just ten miles to the north. At best, this little collection of builder monsters are just getting along, perhaps clearing out an acre or two each year, rebuilding a bit of free stone into a dam or a wall, steadily coordinating their power into something more threatening in the future through systemic defenses and steady population growth. Someday, it might be "civilized" ~ but when the players stumble upon it, we should see the encounter as similar to any other wilderness monster the players might meet. Dangerous, certainly, but not essentially aware of the world outside their little bubble.