The first thing you need to do is go to the Prison in Mintwallin and talk to A Prisoner, to get the key to his room, you need to go to the barracks, south of Markwin. You then need to give 7 apples to A Prisoner to change them for the key to the quest room.
Ask him for the riddle. There is more than one answer to this, but the answer he is looking for can be found in a book in the mad mage room (in a bookcase to the north-east). You need to give the correct order in which a Knight, Druid, Paladin and Sorcerer can cross a bridge when being hunted by orcs. The correct answer is "PD-D-KS-P-PD". You receive silver Key 3666 after giving the apples.
This is the path to the Mad Mage room from the surface. You will walk past this room on your way to Mintwallin.
Follow this path to the east and south, and go down the ladder:
On this level, there will be a lot of Poison Fields. You can either run through them and then use Antidote, or you can use Destroy Field to clear a path.
Go down another level.
Follow this path to the east and north. Be careful of Minotaurs. Go down the hole that is circled on the map. You will face a couple Cyclopes down this hole:
Head south down the left most passage. Go up the hole to a small room and up again to another room. Here go to the end of this passage and down the hole to a small room and down again:
Head south then west then north then take a right heading towards the level door.
Player:hi A Prisoner: Huh? What? I can see! Wow! A non-mino. Did they capture you as well? Player:riddle
A Prisoner: Great riddle, isn´t it? If you can tell me the correct answer, I will give you something. Hehehe! Player:PD-D-KS-P-PD
A Prisoner: Hurray! For that I will give you my key for - hmm - let´s say ... some apples. Interested? Player:yes
A Prisoner: Mnjam - excellent apples. Now - about that key. You are sure want it? Player:yes
A Prisoner: Really, really? Player:yes
A Prisoner: Really, really, really, really? Player:yes
A Prisoner: Then take it and get happy - or die, hehe. Player:bye
A Prisoner: Good bye! Don't forget about the secrets of mathemagics.