Matthew calmly climbs over the wall with Matthias' help to steady him, maintaining the illusion as he goes, careful to avoid what's left of the hot oil.
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taking ten on the climb = 10
Trying to keep alert for a change:
Spot, looking around - (1d20-2)[3]
Listen - (1d20-1)[13]
Seeing Matthias goof up following him he whispers.
Just take it easy. Try again. Forget about the crossbowmen. The leader has raised his arm to tell them to stop shooting for some reason.
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This last is said in case Matthias can't see through the fog.
Matthias should make a DC 15 Will save with +4 bonus to see through the illusion. Or possibly a normal save followed by the save at +4 if he figured the fog was an illusion before Matthew told him.