In Relic Castle you can obtain these fossils, the Plume fossil and the Cover fossil. The Plume fossil has Archen(Rock Flying), and the Cover fossil has Tirtouga(Rock Water). And if you want a Yamask, you can come here too because this is the only place where you can find them. Later in the game, Team Plasma is also there, and when you defeat the Elite Four, you can gain access to the bottom floor, which has a Volcarona level 70.
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