well,go to route 4 and stop when ur close to nimbasa city. you will see a highway.head left,and then up. your gonna see 2 guys standing together go past them and youl be in another desert.keep going up and you'll see stairs going down by these one statues.inside there will be quicksand.if u run u will sink.walk on them slowly but dont touch the middle.there will be other stairs,and your in.
Relic Castle
Relic Castle
Desert Resort.
you don't "pass" relic castle you go to it later and meet team plasma (you also get a fossil if you talk to the backpacker!)
By trade.
if you go down to the very bottom of the relic castle go up the all the stairs and you should be out. hope this helps! =)
there is a backpacker in relic castle who will give you either a plume fossil (which is revived into an archen) or a cover fossil (which is revived into a tirtouga)
relic castle
team plasma does
in the desert resort
route 4 or relic castle
at the relic castle