Hello Pere, and

to GameBoomers!
I haven't played Traitor's Gate, but here is what one walkthrough tells you to do at this spot. If this doesn't help, there is apparently a saved game right after this spot at
GamesOver which you might try downloading: SPOILER
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Get out your crossbow, drag the box of tranquilizing ice darts onto the screen and click on one to load it in the crossbow. Click on the checkmark, then when the guard is motionless next to his guardhouse on the right end of his short patrol path, carefully shoot him in the neck.