As you enter Shadowgate Castle, look to your left and pick up the hollow skull. 

As you do, supplies will be teleported in. Take them.

Use the key from inside the skull to open the first door.

You will see a key on your left. Don't take it. Use an object, such as the scroll found in the chest to hold down the trigger. Then pick up the key. Whatever item you used is stuck there, so just move on. 

Move backwards and open the door you passed with the key. Take the sling.

Move down the hallway and to the left. Pick up the mystic bomb. There's a hidden switch on the wall to the right of it. Press it.

After the secret door opens, move past that entrance to the three closed doors. Take the one on the right.

Continue past the altar and through the room with the "waterfall". Pick up a stone or two, and give the sling and a stone to one character. There's a hidden switch here, press it.

Take the bag with the orbs, and distribute them as you see fit.

Move back past the altar and across the hall. Open the door.

Save your game. This part takes good timing, so if you mess it up, that's it. Take one step into the room and grab the mortar. You'll see a fireball coming at you. Take a step back and close the door. Done correctly the fireball will impact harmlessly off the door. Done incorrectly… load from the save.

Once the mortar is in hand, place the ice orb into the alcove to your left. 

Return to the altar room and place the ice orb upon it. Take the key and the orb. Recharge the ice orb again. 

Return to the secret door you passed earlier.

You'll see a torch on the right and an empty torch holder on the left. Move the torch and another secret door will open. Take the spear as you pass. 

Move forward and to the left, until you see the "wraith". Throw the mystic bomb at it.

There's a scroll in an alcove ahead you can read. Into this alcove, place the lighting orb to be charged. Move past the captured ogre and place the lighting orb. This opens the projection room and you'll see a box teleport in. Grab it and you'll notice a note has replaced the orb. Read it. Loot the chest.

Open the middle door, the only one now closed, and move through the hall of the dead. 

Before you turn left to enter the next section, turn right and press the hidden switch. Recharge the air orb here.

Save and press the right button if you want to be amused by being sucked out into space. Otherwise, continue by pressing the center button.

Throw the ice orb into the chamber while just standing at the doorway.

Place an item on the alcove to the right, just before the pits. Any item will do. Walk across and open the next door.

Use the sling to hurl a rock to the end of the passage. Traverse the passage.

Pay attention to the secret button you will pass on the left. Press it. Nothing seems to happen, but you will press another button that depends upon this button being pressed first, later.

Open the ornate door, and place the wind orb on the alcove to the right.

Turn around, and watch as the heavy statue is magically lifted. Proceed into that room.

There is a button here to the right. Press it twice. Take the west tower key from the secret room it opened. 

Note the hole at the end of this walkway. You'll be back. 

Otherwise, continue to the next room, pressing the button you pass on the right. Return to where all the pits were and pick up the gauntlets and the sack. 

Put the gauntlets on. By that I mean equip them on a character. Loot the sack. Don't drink the water, you'll need it to make an item in a minute.

Proceed back down the hall and to the left.

Pick up the acidic gem and place it into the eye socket. Without the gauntlets on you would have been poisoned. Good thing you followed the walkthrough.

From the secret room take the bracelet (don't get too attached), the holy gem and the other tower key. Open the morar and put in the holy gem and the water. Grind together and take out the holy water bomb. 

Back down the left hallway, open the door to the banquet hall. Ignore the clipping on the rug and move to the right. Proceed up the right staircase and unlock it.

Place the spear in the holder and the wall in front of you will open. Go inside. Read the note on the table (or not, I won't be mad if you don't) and proceed past the red wall hanging. Take the "shooting star" bomb. Turn and pick up the piece of paper with the clue on it. Proceed up the stairs.

The clues up to this point all reference a star for this section, and that is what you must make with the torches in this room. Leave the one directly ahead. You will see two torches on the left wall. Start facing it. Walk to the left and take the first torch on the left wall and move it over to the left. Continue left. Move the torch on the back wall over to the empty holder. The torches now make a star. Hit the button and go into the secret room.

Take the sword, and go back down the stairs to the left set of stairs up.

Open this door and kill the tentacles with the holy bomb. Down the hall, kill the wyvern with the shooting star. Use the gear gear key to open the armory. Pick up the first golden item on your way. 

Loot the armory.

Proceed down the stairs again.

Back in the banquet  hall, move a torch from the wall to behind the hanging tapestry. A door will open behind yet another. Go down the stairs.

Take the right passage. It's your friend the Ogre! Seems his bracelet was stolen and he wants to find it before he leaves. Wait, how did he get down here when I open… don't think about it. Put the bracelet at his feet, and he'll be gone. You can also throw it at him, which angers him and he'll probably kill you. So I wouldn't suggest it. 

Artfully dodge the fireballs into the next room.

Follow the direction on the paper, pressing the button after each line of the clue.

Take the second golden item and make your way down the passageway.

Take note of the boulder to your right as you move down the hallway. It'll kill ya!

Go wayyyyy down the hallway. 

Hit the switch on the wall, then wait for the door to open. Take the blue gem, then the last golden item and quickly make your way into the side room. Wait for the boulder to roll past. While you do, pick up the second blue gem.

You're done here, so move back to the main level. Return to the room with the first creepy statue you saw. Place the gems. 

This will give you the Fire Orb, needed to forge the golden sword. Take it.

Remember that hole I told you to remember? Jump down it. I'm serious.

Heal at the crystal. Place the three golden items next to each other on the right side of the room. The sword goes on the left. 

Place the fire orb on the altar and shazam, you have a shiny new sword made of gold. 

Take it. TAKE IT.

Step through the teleporter. 

You're headed back to the lower level, right before you artfully dodged the fireballs. You'll notice two walls on either side that look out of place. Press the secret switch in one of them, go through the other.

Unlock the door with the ice key you've had forever.

Descend into the underground. 
Save if you want.

As you move through, you'll see a scroll appear. Read it, or not. Continue forward.
You can't get lost.

You'll soon come to a large open chamber where a figure dances about doing the summoning ritual for the behemoth. He'll attack as soon as you enter the room, so make sure you're ready! Only the golden sword will really hurt him. A couple of swings should do it. 

You win!