Desert Resort.
Relic Castle
Relic Castle
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)
in the desert resort
relic castle
In relic castle
at the end of it
route 4 or relic castle
at the relic castle