Opening the big door allows you to fight Tyrelia, who then gives you the means to get past the rock slide to the desert.
In one of the rooms along the long hallway in the fortress, you find an amazon secured in a tickling machine. Free her, and she will give you a hint on how to open the door. If you want, you can: check under one of the training dummies in another room. There is an amulet that is used as a key to open the door, just examine the door, place the amulet in the indentation, and voila!
After you get the earthsphere, go back to the rockslide, and it will disappear to give you access to the next area in the game. Beware in the desert, the longer you stay in there, the more health you lose, so make sure you know where you're going!