You can get to the Relic Castle by traveling to Nimbasa City and going down until you reach the desert. Once you are there, you will see another path turning left. Follow the path until you see it turn. Go up, then left, then up through the Desert Resort Gateway. On the other side, you'll reach a more open area of desert. Keep walking up until you find a stairway leading down, in the middle of nowhere, surrounded by statues.