(Keep in mind, I wasn't that into it, just doing it out of curiosity and the unexpected interest in me completing it - though, now that I know they expected an actual write-up, not just to see my 'build', I consider them slightly less crazy for being curious - so don't expect TOO much effort. If you wanted QUALITY writing, you'd ask me to do a write-up of a CYOA I actually found interesting/inspiring. Also keep in mind: I've no idea why anyone wants to see this, so I've no idea which parts to focus on)

[91] Area Increase 5, [90] Attitude Adjustment, [86] Extrahuman Population, [82] Technology Upgrade 4, [80] Magical Upgrade 1, [79] Gateways 1, [78] Public Education, [76] Aesthetics, [75] Healthy, [73] Diplomatic, [72] Animation Affinity, [71] Nature Affinity, [70] Gaming Affinity, [69] Intellectual Affinity, [68] Mechanical Affinity, [65] Protection 1, [61] Portals, [60] Fertile, [57] Landscape Adjustment, [52] Fauna & Flora 3, [50] Spirits, [47] Lifestyle Upgrade 2, [45] Automatons 1, [43] Immortality, [40] Spirit Walk, [35] Shape Shift 2, [34] Tongues, [32] Memory, [30] Peak Condition, [29] Levitation, [27] Healing, [25] Enchanting, [23] Machina Mind, [20] Nullification, [17] Friendship 2, [42] Bound!, [39] Biomancy, [33] Influence 3, [32] Literature Affinity, [27] Technomagic, [25] Pretty Population, [23] Infrastructure, [22] Occult Affinity, [21] Secure, [19] University, [18] Festival Affinity, [17] Food Affinity, [16] Sanitation, [10] Population Increase 3, [7] Dungeons 2, [3] Metahuman Population, [0] Afterlife

The Wolf Lair Plane spans about five times the area of Earth.
But at the center is 'The Hub', with the twenty Gateways, each wide enough to fit a large city.
Half (specifically the 'Inner Ring' of Gateways) are connected to the Ten Corners, which are matching Gateways spread out through the Plane - So, to travel from one Corner to another, you enter your 'local' Corner Gate, then jump on the monorail (which is hovering magically to reduce friction, of course), probably pull out your laptop (the station sells affordable energy crystals if you didn't bring your own) and browse the web for the half-to-full hour the trip can take (you're going the distance of one or more cities, and the train can't build up proper speed if it has to keep stopping), and then head out the destination Gate once you arrive.
Travel several times the Earth's distance, without dealing with awkward Airport Security issues or lengthy travel-times.
The other half, the Outer Ring, are connected up to ten other worlds selected by the Creator either because they serve as Hubs themselves, with connections to many other worlds in turn, or for their individual significance, like the 'Material Plane' that has the Earth and such.
In addition to the Outer Ring, more temporary portals can be created to link up to other worlds in pre-designated areas near each Corner (as well as between the Gates of the Outer Ring) - the main point of the pre-designation is to avoid the portals being used offensively; No opening a portal to the Plane of Sewage in your rival's backyard, for example.

While the 15-million sapient beings are about 30,000 times the 'bare minimum' for long-term survival (by the 50/500 rule), they're divided across 300 cities of 50k people each, with about 30 different sapient races.
Though several fall into similar, and usually genetically compatible, groups - Like the High Elves and Dark Elves, or the Catfolk and the Elkkin - and each 'group' tends to be located near the same Corner.
The Slimes are unique in their... Well, uniqueness; No other race shares their amorphous shape, or the loose connection of their cells (by which I mean, a Catfolk could, with some effort, push their arm straight through a Slime without hurting them, as the cells are pushed out of the way - though the Catfolk themselves might be hurt, when the Slime slaps them for the offense; Sticking your arm into someone's kinda rude, yaknow?)
Though, some 'cities' are less centralized, having some of their population living out in farming villages or such.
But they're generally within trading-distance of one of the Corners.
The high level of magi-tech already available, and especially the intra-dimensional portals, does limit the social experimentation that can be done; Globalization is already a fact, there isn't much room for the cultures to develop for a few thousand years largely isolated, so that they'd have their own distinct culture.

In terms of culture, it's as close to a Diverse Utopia that the Creator could manage.
War is discouraged by reducing the anger and the 'reward', emotionally speaking, for obtaining physical gain, and then increasing the desire for being seen as 'virtuous' - this does open the natives up to being exploited, as they are also less ready to fight against oppression, but hopefully the increased political interest they're granted independently of this will help. It's also intentionally not going to stop 'defensive' wars; If they are attacked, they should be ready to defend themselves with lethal force if necessary.
Just small adjustments, though; Too much of a change and you'd risk accidentally making a Dystopia instead.
Not that there is much room for intra-Plane war with an immortal Creator still alive and well - one that even has control over the afterlife, and could declare someone unfit to be reincarnated - and that's not even getting into the trouble the various Nature Spirits could cause if people started marching huge armies through 'their' lands.
And while the exact interests vary from city to city, and especially between different Races, you can usually find someone interested in most things tech and food (even technological art, like animation or games), though literature is the only non-tech, non-food culture you're likely to find; With so much to do already, there's no room for music industries getting big, or to establish large galleries of paintings.
Oh, and people enjoy nature, but that's more of a fact than a culture - and might stop being true once they've expanded across more of it and gotten used to the appearance of concrete streets and artificial walls as far as the eye can see.
The magic of the world isn't very strong, but everyone's taking full advantage of it as if it was another law of nature - which it kind of is (or, rather, it's a whole suite of laws) - so they can build stuff that wouldn't work normally, bypass some speed-bumps they'd otherwise hit, and so on to improve just about every part of everyday life.

For the adventurous, the un-tapped areas of the world are filled with strange, magical beasts, and hidden dungeons with treasure for those brave enough to get past the obstacles.