- "Power- now all to droplets turned in the temple of Arche Koeln."
- — Epitaph of Twilight —
Arche Koeln Waterfall (アルケ・ケルン大瀑布, Aruke Kerun Dai Bakufu) is a field located at Δ Hidden Forbidden Radiation. It is one of the Lost Grounds and its name means "The Land of Origins".
When humans were fighting against the "Shadow", this is where a young apprentice magician called forth the gods.
Arche Koeln lies in the "Land of the Beginning of Beginnings". Within the Temple of Arche Koeln, past its waterfall seal, is the entrance to the path leading to the End of Land. This is also mentioned as the place to which power returns.
Arche Koeln serves as a meeting ground for the Twilight Brigade. Ovan, Shino, and later Haseo frequently visit the area to discuss the affairs of the Guild. At one point Tabby, Sakisaka, and Haseo were teleported here after encountering an active sign created by Tri-Edge. After Ovan's mysterious disappearance, Shino and Haseo spent a great deal of time in the area, doing nothing more than sitting in silence. Tri-Edge's A-shaped Sign is clearly visible on top of one of the bridge supports. After Shino was placed into a coma Tabby began to frequently visit the area for meditation.
When Ovan finally returned to The World he asked Haseo to meet with him here. He informed Haseo that he could meet Tri-Edge in Hulle Granz Cathedral and that they should go there together. Too impatient to wait for Ovan Haseo went ahead by himself and attempted to fight alone. Without Ovan's help Haseo was easily defeated and data drained by Tri-Edge's Twilight Bracelet. Shortly afterwards Phyllo came to the area to meet with Ovan. Ovan revealed to Phyllo the truth about what was going on in The World, a secret Phyllo promised to take to his grave.
Arche Koeln is mentioned in the official mythology of The World R:2 on CC Corp's website. Ovan e-mails Haseo asking to meet with him here. When he arrives Ovan dodges Haseo's questions about his activities and tells him where Tri-Edge will appear next. Eager for revenge Haseo prepares to travel to the Hulle Granz Cathedral to meet Tri-Edge, with Ovan promising to meet up with him later.
- The keywords for this Lost Ground in the Japanese release and the English release of Rebirth were Δ Hidden Forbidden Waterfall, but were changed for the American release of Reminisce. The reason for this change is unknown, though it may be the result of a homophone mixup. In G.U.+, Tokyopop used "Waterfall".
- This Lost Ground's name is a combination of the German word Arche (ark) and Köln, the German name of the German city Cologne. Thus, the name means ark Cologne. "Cologne" in turn is derived from "colony."