Well there is the theory that Stone Tower was kind of like the Tower of Babel. I assume the ancestors of the Terminians built Stone Tower. So the portal might lead to what was once Termina's Sacred Realm. My theory is they built Stone Tower with the intention to use the Triforce to kill the gods. They didn't succeed, the gods revoked their privilage of having a Triforce, and abandoned Termina all together.
That's an interesting theory, given the noticable lack of Triforce in Termina... except for its depiction on the pillars leading to none other than Ikana Canyon.
This would also explain the lack of any deity in Termina except for the giants. Majora even may have been created to contest the gods, or to serve as a deity in their stead.
But what does any of this have to do with the battleground where Link fights Twinmould?
If we think of Stone Tower Temple as the Tower of Babel, and this area exists at one end, then given that the tower flips - which end is it on? The top or bottom?
Mayhaps this place is the remains of Termina's sacred realm - or its closest counterpart, at least - all dried up and abandoned after the gods deprived Termina of its triforce and bounced?
To tack onto the Maskmans post about the Ikana tower being the tower of babel, when the tower complete the gateway to the heavens was inverted and so lead instead to hell, in this case we can see the similarity between where the Garo Master jumps down from and the gateway to Twinmolds room. Another theory also claims that the room where Twinmold resides is a pocket reality where Majora's Mask resided for some time before being claimed by the Happy Mask Salesman.
Ikana itself has a whole undead warrior theme, which'd match with ancient beliefs that a dishonorable warrior wouldn't be able to enter heaven, and seeing as the ikanians seem to shun the light and gods and embrace darkness makes sense.
Termina never seemed to worship the triforce in fact according to one theory (that self same tower of babel identicality theory) The Ikana tower was built as a blasphemy to the goddess' The original version depcited statues lick a triforce located on their rears showing disregard for it's power, the phallic statues that point to the sky and the obscene gesturing by the massive statue all point to Ikana having a beef with the gods, the gods then chose to turn the temple upside down with the power of light meaning any warrior who sought to climb it to fight them would instead descend into hell (SS did this better with the Ancient Cistern). We've been shown theres a sort of "spirit" plane in Termina via the realm we enter on obtaining a boss' remains or a mask from Mikau and Darmani, even the Moons interior is shown to be another plateau, so in that vein i think perhaps Twinmolds location is similar to Demise's a being of great power existed there and folded reality to suit it's need but when it left it withered until it had no purpose at which point it was appropriated by Skull Kid and the Mask.
Side note, why has no one ever looked at Majora's Mask and considered that it was all a trial for link? To prove that once more he is the Hero? If we look at OOT Link he was blessed by the gods and given great power he descended to the depths of the earth and to the tops of the world (figuratively, in this case referring to the temple collosus and the tower of ganon's) and yet when he left Hyrule he was stripped of all that made him a hero, If we take termina to be a parralel of hyrule why is there no terminian link? WHy in fact is Skull kid the very some one who link once gave a mask to? Clearly some time has past for Skull kid to have forgotten him (or maybe terminian time is vastly different) But we see link scale mountains, enter poisonous swamps, go to the depths of the sea and even descend to a metaphorical hell, in the end he ascends to the heavens and fights what can be construed as a false god, he even can obtain a "divine" form one that seems similar to his adult self but vastly more powerful. Link could've given up the quest at any point but he saw the plight of termina and stepped up, he coulda just bailed once he freed epona but nope.
Apologies if this seems long and "rambly"