I know it is not Zork or Wizardry, I found both of those and remember playing them but not the one I am thinking of.
I also remember another room where there was a crystal ball and after you broke it (can't remember how you break it) Merlin appeared and gave you some kind of hint.  I sort of remember another room where a vine or something comes at you and you have to use the knife you picked up in a earlier room to get pass the vine.  If you don't it sends you back to the beginning.
I checked out some screen shots of Dark Castle that Slycrel linked to earlier and that isn't the game either.
Maybe it is just a sick dream I had.  The worse thing is I have found every other game I remember playing except this one.  Anyone remember playing the math game Gulp while in school?  I had the highest score for the longest time in Elementary school on that game.