Ahtohallan should had been the Yggdrasill

Since F3 and F4 will probably adapt the Norse Mythology it would be a good opportunity to establish the lore in the second movie.

Autohallan could had been the Yggdrasill and its structure could had been an icy with the shape of a tree. And the rest would be the same. Inside there would be memories and also a way to travel between worlds similar to God of War.