Castle of Pain & Sorrows
You've made it to Mordred's castle, but before fighting Mordred you need to take care of a few hurdles that stand in your way. Fight back the forces in the castle, free the Quest Knights, locate the Temple of Evil housing the Minister of Evil, Argantan, discover the Scroll of Diablo. This level has perhaps the most eye candy that you are not required to visit to complete the level, so I recommend exploring the level just for fun.
Follow the underground passage and take the platform up to the Castle of Sorrows
Go to the left, and you'll trigger a secret door housing a future pod with recharge, save buffer and terminal
Continue down that corridor, turn right, then turn left (leads you to an open area split by a deep water passage)
Enter through the door on the left, cross that room and enter through the door on the other side
Take the door on the left and flip the switch in that room
Go across the hall through the door that just opened and stand on the pedestal (that opens the door to the Hidden Keep)
Enter the keep and grab the key from the last small room
Back track to the corridor where the future pod is and save the game
Now take the corridor which crosses the courtyard and moat, and turn left at the end
Go down a short ways, then turn right to face a small alcove with a switch in it
You can avoid an ambush by using a weapon to activate the switch (this switch opens the secondary gate)
Turn around and go back past the corridor and turn to the left (there's a door on the right and a drop hole on the left)
Jump into the drop hole, approach the flag on the wall, and a secret door will open
Pass through the secret door and turn to the right to enter the inner courtyard
Go to the right and flip the switch on the wall
Now go back to the left and use the corridor passage to enter the guard house (flip both switches in the guard house)
The door across from the guardhouse door is now open, take this corridor and open the first door on the right to free 2 Quest Knights
Now enter the room across from that cell and go down the stairs to the dungeon
Flip the switch to enter the dungeon, then go all the way to the back and open the door on the left
Flip the switch in that cell (frees all prisoners and opens a secret passage back up to the inner courtyard)
Take the secret passage back up to the courtyard
Once again go down the corridor behind the guard house, and this time follow it all the way to the end
Past the room with a table is the throne room, enter that room and climb the stairs along the back wall
Go behind the wall behind the throne and use your key to unlock the lock (this opens the door to the Temple of Evil)
Enter the Temple of Evil and either kill or avoid the Minister
Go to the back of the tempe and read the terminal to move to the next level
Eye Candy / Extras
Jump over the wall onto the courtyard near the beginning to collect some extra ammo and a 3x potion
To the far left, jump across the moat to the doorway and take the stairs behind the door up to a switch which raises the drawbridge
Cross the drawbridge and you'll find a switch on the right which opens the primary gate
In the hallway between the primary and secondary gates are two walls with flags which are both secret doors
Facing the secondary gate, step up to the flag wall on the left and climb those stairs up to the room with the fire pit
Across the room is a door to a bedroom, and on the bed is a magic staff
In the corner of the bedroom is a door to a secret room containing water vials and a 2x potion
If you exit the fire pit room through the big wide door, you'll end up back on the West Hall (easy access back to the future pod)
If you jump into the moat and go to the west, you'll find an alcove with bones that will teleport you to the West Courtyard
If you take the moat to the east and follow the tunnel all the way, you'll reach 4 bars that can be opened with the action key
Pass through the bars and swim to the top, and you're in the bath area of a room south of the Upper Courtyard
Another way to get out of the moat is to swim up right before entering the underground tunnel, and there is a pebble rock staircase back up to the West Courtyard
From the first pod area, take that corridor to the east to the South Tower, then turn left and look for a switch on the right wall, flip that switch to open the gate to the Upper Courtyard
Now jump down into the West Courtyard and jump across the moat to the doorway by the pebble rock stairs
Climb the stairs up to the Upper Courtyard (by the way, an easier way to get to the Upper Courtyard is simply to go through the door near the switch you just flipped to open the lower gate)
The 2nd door in the Upper Courtyard leads to a south guard room
The first corridor on the right ends at a secret door back out to the South Tower
The corridor in the far end leads to the bath area (the bath drops down to the moat tunnel)
The corridor in the right wall leads to a bedding area where you can find some arrows
The stairway immediately to the right leads up to the balcony where you can grab the 1x potion and oxygen potion
The passage past the balcony continues around to the north side of the Upper Courtyard, and halfway down the stairs on the other side is a window to the courtyard
There are two nooks in the walls as you head towards the littel window room, and if you stand in the nook on the right, it will open a secret room behind the nook on the left revealing a 2x potion
In the little window room is some water vials
Continue down the stairs and you'll end up in the north guard rooms where you can find a 1x potion (the north guard room exits to the West Hall)
After freeing the prisoners in the lower dungeon, there is a 2nd future pod revealed in the inner courtyard with a 2x recharge, save buffer and terminal
Walk towards the north east corner of the inner courtyard to reveal a secret door to the east guard rooms
Follow the secret corridor, climb the stairs, and you'll find a 1x potion in 2nd room
Head back to the first room and exit through either door to the balcony in the banquet hall
Jump down onto the banquest table to grab the 2x potion, then exit through the big doorway back to the inner courtyard
Next to the seconday gate to the inner courtyard is a door which leads down to the skull snake pit (contains some arrows and a 1x potion)
A door opens upon entering the pit, and if you step into that room, you'll be teleported to a secret alcove behind the stain glass in the banquet hall