Maybe you were using an outdated walkthrough version? The solution for the assassins guild dungeon is included in the FAQ section at the very bottom of the walkthrough.
Since last update, you also have the consumable item "Riddle Key" to skip dungeons when you get stuck. (Limited to 4 at the moment, to make sure people try the riddles before skipping them)
Items can be used in battles, not just skills.
The clock tells you which path is open, counting from the left to the right.
