Theory: deep af
Nintendo: it's just a temple with rooms full of traps

I've played OOT a lot (like many others I'd imagine)  but I never realized just how cool the layout and architecture of the forest temple was until those panoramic shots from this video. Nice work Zeltik!

It's Ganondorf's college art/architecture project. He thought adding all these wacky passages would make for a modern art piece.

This actually makes a lot of sense when you consider the fact than ganondorf left his phantom to guard it. Not some monster he unearthed, or dragon he revived, but a copy of himself. Obviously he felt it was important enough to require his presence, or at least a close enough facsimile of it.

The boss room probably had paintings of the sages that predated the OoT sages.

Me: "Oh, nice, a new Zeltik video"
Zeltik: "The Thicc -"
Me: "This is gonna be a good one, I can tell"

I always felt like the forest temple was far more unsettling than the shadow temple, especially with the music. But then I felt like Snowhead's desolate isolation was way creepier than Ikana canyon too.

I’m intrigued by how the Remake will treat this iconic dungeon.

The fact that the Poe sisters are named after the little women just shattered all of my preconceived notions of reality.

9:26 perhaps it's something like Windsor Castle?technically a fortress but more meant as a home for the royal family and used as a retreat for hyrulian citizens in a pinch

Me: "well, it's time to go to bed"
5 Minutes Later
"...the undead enemies inside may be the deceased, reanimated soldiers from this encampment..."

I always felt that during the events of Ocarina of Time when Link was trapped in the sacred realm (or whatever actually happened to him for 7 years) after pulling the Master Sword, that the Forest Temple was used as a hideout for Zelda and Impa for a few years, until Phantom Ganon took it over. Hyrule is rather small, and there’s only so many places they could hide.

The Gerudo are at least connected to Ganondorf, if not allied with him, so they’re out. The Castle and Castle Town are in ruin. The Kokiri Forest and Lost Woods are protected by ancient magic that wouldn’t let them hide for long. Lake Hylia is drained, and Zora’s Domain is frozen over. Kakariko is too close to Ganondorf, and too small a hamlet to effectively hide for long. The Gorons have been torn from their home and imprisoned, while Death Mountain spews fire, hinting at imminent destruction.

There are small areas where Impa and Zelda can shelter and hide in, but none for any length of time. And Zelda would have needed dedicated training in order to become Sheik.

The Triforce room at the bottom of the Forest Temple is idyllic. It’s a hidden sanctuary, it’s easily guarded and fortified, and would make an excellent place to escape to when Ganondorf usurped the throne.

Not only that, but Impa rides off directly towards the forest region of the map when fleeing Ganondorf.

I do not disagree that this temple was built as a divine place to reach the sacred realm, but it’s original use is long forgotten. During the events of OoT, it was the primary safe house for the royal family, IMHO.

The Forest Temple music is my favorite.

I know the entire video was to deal with the in game lore and purpose of the Forest Temple, but may I offer something more of a meta hypothesis that just might help the pieces fall into place?  It’s common knowledge now a days that OOT was originally going to mirror Super Mario 64 in the fact that you’d have a central hub where you would traverse to the different places via the use of portals  like how Mario did with the paintings.  The reason for that would to save on RAM space and make the game look more pretty.  Miyamoto didn’t like that idea and scrapped it sometime early in development for the more open world that we ultimately got.  The team did pay homage to the early build by including Phantom Ganon popping in and out of those portraits.  So, what if the Forest Temple was originally the HUB Hyrule Castle and the room where you fight Phantom Ganon was supposed to be where you got the Master Sword?  That would make sense, right?  You can even use that to help with in game lore.  There are several iterations where the hero gets the Master Sword in the Lost Woods, the place where the Forest Temple is located deep within.  So what if the prized possession that they were trying to hide/protect was the Master Sword?  Think about it.  Yes, the temple itself was located high above, but the room was located in the basement, which would put it roughly even with the forest floor.  What if they built the temple above the chamber and had the puzzles, traps, et cetera be the trials that the hero had to finish in order to get the four flames, expose the elevator, solve one final puzzle, and then claim the sword.  But then, despite all the traps and efforts, due to the curse of Demise, some force of darkness infiltrated the temple, destroying the steps, reeking havoc inside, and managed to get to the final chamber, only to be stopped by Rauru himself whom then proceeded to build the Temple of Time, moved the Sword (he is the Sage of Light after all) sealed the door, and gave the gems/keys to two of the other sages and the Great Deku Tree, knowing that he couldn’t give all three to three different sages lest the forces of darkness do exactly what Ganondorf did.  But hey, that’s just my take on the possibility.

What's interesting is that the Great Deku Tree track is calming which makes the dungeon feel more 'homely' and 'natural', as it is inside a real, living tree, and in located in Link's home in the forest, while the Forest Temple track is made to sound unsettling and makes it feel more 'unnatural' and out of place seeing as how it's a manmade building in the middle of the forest; not only that, but the fact that the Great Deku Tree goes underground, which creates the feeling of being connected to the Earth and reassured/safe, while the Forest Temple is higher up off the ground, which makes you feel disconnected from the Earth and 'scared/unsafe' as well as inducing a general fear of heights/falling.

I always found the Forest Temple to be generally creepy, but then I really thought about how unsettling it is in comparison to the Great Deku Tree.

My theory: The Forest Temple was an entrance to the Sacred Realm but sealed shut to minimize the Intruders. Afterward it became a Safe House to hide the Royal Family, if all obstacles were overcome then they escaped to the Sacred Realm to safe but only as a last resort by the look of the defensive steps taken. It was a safe haven to escape to during wartime. Which explains Links Arrival in the Kokiri Forest. Wow you some amazing insight.

Personally I always suspected/assumed the temple was an ancient villa or vacation retreat for the Hyrule royal family that had been abandoned then claimed by the deku tree, lost woods or kokiri to become the temple for the sage of the forest.

This temple traumatized me as a child. As an adult I still get creeped out every time I play it. For some reason I always feel like I’m being watched, especially in the area with the large blocks and ladders, before the first twisted hallway.

Considering in most Zelda games the master sword is typically located in the lost woods, could the forest temple have originally housed the Master sword?

I also like to think the art direction or theming of the temple was inspired by Edgar Allen Poe and the Fall of the House of Usher. (but I definitely think you're right about the temple's meaning within the lore and timeline). Mainly the crumbling mansion from that gothic revival architectural era, the name poe, the ghosts being sisters, and the theme that the family may have caused their own downfall.