|Weiss, you dumbass!|
SPOILER ALERT - Plot details for NIER follow.
The Shadowlord's Castle is a location reached via a magical portal located on the top of the Lost Shrine in NieR RepliCant and NieR Gestalt (NIER). It is where Nier and his friends, Grimoire Weiss, Emil and Kainé fight the Shadowlord.
It appears to be not actually a castle but a 21st century office building in a large city (most likely Shinjuku), which has been preserved by the Shadowlord and sentient Gestalts over the many centuries. In fact, it would appear the entire city is well kept inplying that many Gestalts are sentient and have not relapsed as Replicant Nier would believe from their hostility in his small corner of the world.