The Relic Castle is the ruin of an old castle situated within the desert with two entrances. One in the west of the desert doesn't allow much access while the eastern entrance is just a stairway in, surrounded by Hihidaruma. Over time, it has been filled with sand and navigation throughout the various levels of castle is done over and through various pits of quicksand. To traverse this quicksand, you need to walk on them slowly. If you run or bike over it, you will be dragged down to the next floor. Team Plasma arrive in the Relic Castle in order to look for the Light/Dark Stone in Black & Whites. However, when the Pokémon League has been beaten, a new maze at the very bottom of the castle opens.
The way back up from the end of the maze match the wild encounters on the equivilant floors
In Black 2 & White 2, the Relic Castle is seperated into two parts. First is the top part as accessed by the Desert Resort, which only comprises of the top two floors while the bottom part is accessed via the Relic Passage.