[Radek]

Nokturnal, ALL your spells will fizzle until you kill the Boogre Guard near the regular entrance to the prison. More precisely, all your spells are disabled after entering the prison, no matter whether you enter through the backdoor or through the regular entrance, and they remain disabled until you open the door to the "cage".
The guard opens the door normally. If you kill the guard then the door opens as well (the guard is the trigger for the door and the game follows the rule "killing a trigger triggers the action", so that killing the guard opens the door).
Therefore, you must go to the entrance without your spells. You will see a "--|" there, where the horizontal branch goes to the cage. You will come to the lower branch. The guard is standing at the upper branch near the way to the cage. He identifies himself as a "Boogre Guard" (the only boogre guard in the whole dungeon). Kill the guard and your spells will be reenabled.

Here is what I am always doing in the prison:
1. Enter through the backdoor.
2. Kill the lillies.
3. Leave the prison through the backdoor, heal, recast spells, reenter the prison.
4. Kill the boogres near Priskiella.
5. Kill Priskiella. This opens the door to S'Keser Da without the ring.
6. Kill S'Keser Da and get the Evil Eye.
7. Investigate the teleporting maze (it does not function now).
8. Leave the prison through the backdoor, heal, recast spells, reenter the prison.
9. Go to the boogre guard and kill him. Your spells are reenabled.
10. Run to the Idol of Aku without allowing (the second copy of) S'Keser Da to curse you. Kill the lillies there.
11. Allow (the second copy of) S'Keser Da to curse you. This removes the second copy of S'keser Da from the prison.
12. Go to the idol and get rid of boogreism.
13. Do the rest.