Rules FAQ:
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The STR bonus applies even if you do not have any defending characters (unlike The Red Keep). It can prevent an unopposed challenge, and multiple Haunted Forests can prevent you from losing a challenge (you your total STR bonus is greater than the attacker's total STR, then the challenge would end with no winner or loser), but in order to actually win the challenge you need at least one participating character.
- Note that the Forced Reaction triggers if you lose any challenge, either as attacker or defender.