One of the spoilers say that you must use magic to open the secret door
. There is another way . It works and completes the score but sometimes
it is fatal . Go to the archery place ( where there is a bull like
drawing on the board and where you can practice throwing) at daytime
(forgot what time) and you will see the dude at the town gate ( the
thief) talking with a brigand.They will talk about the password. Save
the game . Walk to the middle archery place ( you have to walk around).
Sometimes the bad thief will catch you so its purely luck . Then when
you reach the brigand at the archery place , kill him and search his
body to get the key to the secret door . Open the secret door . Talk to
yourself or the door to say the password . If you say the password , the
troll will disappear . Then follow the passage and you will land to the
front door of the brigands fort . Another way is picking the lock using
the toolkit , if you are a thief . This will increae your lock picking

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