Its only use is to have an Ancient Key to be placed in its socket, making the Atrium's Nightmare Cycle be locked at the peak state. It can be removed again, unlocking the Cycle from its peak. If a fully-constructed Odd Skeleton is revived near an activated Ancient Gateway, the fight against the Ancient Fuelweaver will begin. The Ancient Fuelweaver can deactivate the Atrium's Nightmare Cycle by removing the Ancient Key. If the Ancient Fuelweaver is killed it will trigger an explosion that will activate after three minutes, renewing the Ruins. Twenty days later, the Gateway will accept the Ancient Key to be placed again.
“That's how we got here. Hopefully how we get back, too.”
In The Forgeevent, the Gateway in the Forge dimension shows up as the starting point where players and their allies spawn at beginning of the game. The Forge's Ancient Gateway is technically a separate structure sharing the same name as the one located in the Atrium. It currently has no other function, but the game files reveal that it also has an active state, which might be used in later versions of the event.
When first released, the Ancient Gateway had black and yellow tapes all over it, to signalize it was still a work in progress feature. During this time, various Items were able to be socketed into it, such as the Iridescent Gem. Picking it back out always produced an Ancient Key. When examining an Ancient Gateway with a key inserted into it, the player character said: "The lights reveal ancient text: Come Back Next Update."
The Gateway is featured in the What Lies Beyond? video clip, for a brief moment revealing a silhouette that is suspected to be Winona.
The "Ancient Gateway" found in The Forge seems to also be called the Molten Gateway in the game generation description. It is unknown if this was its original name before being changed, or if it is what the structure was meant to be called later on and the in-game Gateway was simply not renamed as well.
The work in progress visual of the Ancient Gateway.
The Ancient Gateway's textures as found in the game files when it was first released.
The Gateway after the Ancient Fuelweaver dies.
The Gateway exploding.
After the explosions.
A hidden image of the Ancient Gateway alongside the Ancient Fuelweaver in the A New Reign trailer.
The Ancient Gateway, as seen in the What Lies Beyond? video clip.
The Ancient Gateway, as seen in The Forge cinematic.
Charlie by the Ancient Gateway, as seen in The Forge cinematic.
Concept art of the Forge version of the Gateway from Rhymes with Play # 167.