When you first go in there is a backpacker near the falling sand and talk to him:
Cover fossil=Tirtouga
Plume fossil=Archen
The only fossils that can be found in the Relic Castle are the Plume Fossil which is the Archen Fossil and the Cover Fossil which is the Tirtouga Fossil but you can only select one.
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)
Get the cover fossil from a lady in Relic Castle the take it to the woman who revives fossils in Nacerene Museum.
No one. Your first fossil is given at Relic Castle, further fossils are given daily at Twist Mountain.
They let you pick from two different ones inside of relic castle. Ones a seaturtle and the other is a dinosaur bird.
Relic Castle
go to the entrance with all of the statues, go in, then go to the the third hole and there is a backpacker with two fossils:cover (tirtouga) and plume (archen).
no it is impossible to get both fossils in mt.moon sorry =(
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!)
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! =)