I wanna get the fossil that tranforms into a Tirtouga. I know it is located inside the Relic Castle but I don't know its exact location inside the castle.
Its right inside you tlk 2 the ppl and 1 lady lets you choose either plume fossel (Archen) or cover fossel (tirrouga)
No, you don't have just one chance, as you can say no, there's no other way to get the Unova fossils, and you restore them in the Nacrene city museum. It's on the Ist floor of the Relic Castle, over to the left a ways.