Actually, I thought the God Beasts are stuck guarding the entrance of the fortress so they'll never leave it unguarded no matter how you try to lure them out. Someone would've tried baiting out the Beasts even before LB discovers her abilities. It's possible to lure them upwards I guess, since the Phoenix did chase Haru right up into space! So that means the Beasts is stuck to a within the range of a certain x y coordinates, but can be pulled infinitely at z direction. Quite convenient if you can fly and not die.