A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
A player, 23 March 2008
Castle Walls Castle If you are the Defender and have only one Province, your Personalities and Followers have +2F. Bow this card: Produce 2 Gold, or produce 3 Gold if another Castle Holding is producing Gold for the same cost.
Prepared Defense Siege Terrain Battle: If you are the Defender: Straighten each Castle Holding you control. You may use abilities and resolve effects on Castle Holdings as if you had exactly one province and as if the current battlefield was at your last province.
If I play Prepared Defense, do my personalities and followers receive the force bonus from Castle Walls? Is the force bonus from the trait, an effect that resolves?
Brook Cunningham (rules official), 23 March 2008
If I play Prepared Defense, do my personalities and followers receive the force bonus from Castle Walls? Is the force bonus from the trait, an effect that resolves?
Short answer: Yes.
Long answer:
First, let's look at what it means to "resolve" effects:
http://rules.l5r.com/Glossary_R#Resolve: "An action or trait begins to resolve when its sequence of effect activation begins, assuming costs, targeting and conditions have been met. It stops resolving when its sequence of effect activation has ended, even if the application of some effects has been delayed.
References to an action or trait's "effects [plural] resolving" are equivalent to the action or trait itself resolving."
But continuous traits aren't as cut-and-dried as actions. Let's see if we can figure out when their "sequence of effect activation begins":
http://rules.l5r.com/Effects#Duration_of_effects: "...if a continuous effect has a condition on it the effect begins at any point when the condition is met and the relevant cards are in play..."
So, you have Castle Walls' continuous trait sitting around waiting for you to be the Defender with only one Province in order to activate. You're the Defender with multiple Provinces (halfway there) and you play Prepared Defense. It says "You may... resolve effects on Castle Holdings as if you had exactly one province..." The Terrain's quoted effect activates the trait, since activation of a continuous trait's effects is equivalent to beginning to "resolve" them (the exact wording of Prepared Defense).
Keep in mind, should the Terrain leave play, its effects end (Glossary definition of Terrain), one of which being your permission to resolve Castle Walls' trait as if you were meeting its conditions, so the "resolving" of that trait will end also:
http://rules.l5r.com/Effects#Duration_of_effects: "Continuous effects from cards upon other cards normally end when one (or both) of the two cards leaves play or when the effect's condition is no longer met."