Ancient Gateway

“Now where might that lead?”

–Wickerbottom, when examining an unactivated Ancient Gateway

The Ancient Gateway is an indestructible Structure exclusive to Don't Starve Together, introduced in A New Reign. It is located in the middle of the Atrium Biome.

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.

The Forge

“That's how we got here. Hopefully how we get back, too.”

–Wilson

In the Forge event, 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 in normal DST. It currently has no other function, but the game files reveal that it also has an active state, which might be used in the official version of the event.

Trivia

In the Heart of the Ruins update, when examining an Ancient Gateway with a key inserted into it, the character said: "The lights reveal ancient text: Come Back Next Update."
When first released, the Ancient Gateway had black and yellow tapes all over it, to signalize it was still a work in progress feature.
When the Gateway was first released, various materials were able to be socketed into it, such as the Iridescent Gem. Picking it back out produced an Ancient Key, regardless of what went in.
WX-78's quote, "ERROR 502" references the actual Error 502 which means "bad gateway error".
The Gateway is featured in the What Lies Beyond? video clip, for a brief moment revealing a silhouette that is suspected to be Winona.