Its only use is to have an Ancient Key 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.
In a lore sense, the Ancient Gateway acts as a portal to what are known as Gate Realms. These Gate Realms are the settings for special limited-time Events.
In The Forgeevent, the players spawn in a plume of flames via the Molten Gateway into the Forge Gate Realm. Players must win against the minions of Battlemaster Pugna in order to activate the Gateway with the Ancient Key and leave this Gate Realm. The Molten 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.
In The Gorge event, the players spawn in a gush of water via the Mossy Gateway into the Elder Bog Gate Realm. Chronologically, this event takes place directly after The Forge, meaning that entering the Molten Gateway took the characters straight to this dimension. Players must cook food to satisfy the Gnaw and gain three of the Gnaw's Favors, which are used to charge the Ancient Key and thus open the Mossy Gateway. The Mossy Gateway is technically a separate structure sharing the same name as the one located in the Atrium.
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, 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 of Winona before her official reveal.
The Ancient Gateway located in the Ruins area of the Constant is also called the Atrium Gate.