When I'm standing behind a barricade defending and a monster attacks/wins the duel. If the monster decides to power attack, with say hurl or rend, does the monster still make the required "in the way" check for the wall?

If memory serves a barrier is dependent on the height of the combatants so most monsters shouldn't be bothered with walls and the like. It might be only for the defender's height though so you'd need to check that as I don't have the book to hand.

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"Draw your sword with a heavy heart, but swing it with a heavy hand"

Does anyone have a reference, I couldn't find anything

Pg. 43 of the big rule book. The paragraph right under the "Barriers: hedges fences walls, etc." heading. The important bit is that "a Barrier must be at least half the height of the attacking enemy".

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"Draw your sword with a heavy heart, but swing it with a heavy hand"

Awesome, thanks!