So, first of all, let's situate SP in the world; The world itself is a bit of a fantasy version of earth, where you have all the normal places (Europe, Asia, Americas, Africa, Oceania etc...) coexisting alongside any and all fictional places people put in their canons (think Crank's islands, for example). In this world, aliens, demons, angels, and people with special abilities all exist along one and another. That is not to say that they all get along peacefully, and as a result most people with extraordinary natures tended to keep such things hidden to themselves, that is until the inception of Sierra Pines.
When multimedia started booming in the world, certain entrepreneurial individuals decided that perhaps making all these supernatural beings fight each other in bloodsport would make for a bloody good show. And it did, people ate it up and the business soon exploded. While the practices weren't exactly legal back in the day and had to be broadcast covertly so as to not have a bunch of the authorities shut it down, eventually the social stigmatization faded as the show itself became more commonplace in households. With all the money they made from the show, the RHG corporation quite literally simply bought Sierra Pines, a derelict backwater town back then, and turned the place into a playground and nexus for all such sort of extraordinary people.
So Sierra Pines is in fact a completely independent city state-esque corporatocracy owned by the RHG corporation. Many such figures of exraordinary nature flock towards this place for reasons similar to Richard's Cloudfell; The city is a place where these individuals can feel at home, no longer stigmatized by society, and where they can make a great career utilizing their special abilities, as either someone assisting in maintaining the city's adminstration/infrastructure, or as a gladiator. Of course, there's plenty of normal people that have made it their home too at this point, this is because of the very hands-off approach of the RHG corporation and incredibly low tax rates and such things that are highly attractive towards people with a less stable economical situation. Many who have come here have managed to quite literally rise to the top from nothing, though the city has also destroyed an equal amount of careers with all the lives it claims in collateral damage.
Even then, death as a result of gladiators has become more socially accepted, to the point where it is similar to death by car accident; You know it could happen to you, and it'd be bad luck, but there's nothing you can really do about it so you try not to think on it. The nature of the RHG itself has also changed throughout the year to become more lenient in the amount of lives it claims; While battles with fatal endings are still commonplace today, due to the large populus of kinder hearted supernaturals, and the highly advanced healing technologies the RHG possesses nowadays, they are no longer the industry standard like they were in the past, and we see many more fights happening in good spirits than ever before.
Location wise, Sierra Pines itself exists in an unspecified location somewhere inside of this fictional Earth. It could be somewhere in Europe, somewhere in the US or Canada, but I myself never really defined just exactly 'where' it exists since that'd make it harder for my own canon to peacefully coexist with the others here, hence I keep it vague on purpose. Neither is it this pocket-dimension either, though; Sierra Pines is a very concrete megacity in a very concrete location in the world, it just so happens that I've never specified said location. Though one could make a point that, because of all the interference of superdimensional beings such as demons/angels/younameit, the veil of reality has worn especially thin in this specific city.
As for simple geography, Sierra Pines is located next to a bay that connects it to the rest of the world's oceans. It is a large, sprawling port city big enough to rival even the largest cities in the world. There's big areas within the city designated specifically for fighting, and other areas the corporation tries to keep the fighting away from (think residential areas and administrative areas). Towards the outskirts, the city turns more derelict, with empty industrial complexes beginning to rear its head as prime locations for fights. Beyond that lies nature in all its forms; Meadows, mountains, forests, you name it. These locations are constantly manipulated through weather controlling technology to vary things up every now and then, turning the meadows into deserts or frozen tundra seemingly at will. The city itself is spared such extremities in weather diversity and instead the technology is used to emulate a healthy cycle of the seasons there, with hot summers and white winters.
Ultimately, Sierra Pines is a nexus for the supernatural, and as a result also a great place of interest for many people with hidden agendas, be they illuminati-esque organizations acting behind the shadows or the chief political interests of nations such as the US. The advent of such hyper-developed technology and extraordinary abilities in SP fosters a great deal of medical and militaristic interest in the city.
I think that's about all the world building I have for my canon. Just know that I do oppose the pocket-dimension agreement, though that's simply because of personal taste. I'm not one who is fond of using extradimensional bullshit to explain certain things about my universe.
PS; This is my 299th post. I'ma do something a bit more special for my 300th so don't expect to hear from me until it's done.