Out of curiousity, did the fact that the player had the ability to cast falconskin increase the odds of success on their check(s) to escape?
No, because there was no setting of "a chance to escape". The PC is put into prison. The door is not one a falcon can fly (or squeeze) through. So the PC instead waits to see if his cleric friend, whom he thinks is the sort of person who would go into the prisons trying to help the suffering, turns up: mechanically, the player declares a Circles test (with a bonus, because the existence of this friend has already been established in a prior episode of play).
The check fails, so instead the only person to turn up is the magistrate, who decides to let the prisoners rot there indefinitely (the ingame logic of this I spelled out upthread; the resolution logic is that the player has made his roll to see if people turning up will help his PC escape, and has failed, and so now either (i) some other means will be required (he worked on the assassin who was also in the cell), or (ii) he can wait and see what I do as GM (but my player tends not to like waiting to see what I will do as GM, because he tends to suspect - rightly, most of the time - that it won't be good for his PC!).
it sounds like the player's ability--which should be excellent for escaping prisons--was instead rendered entirely useless in that circumstance.
I'm not sure why being able to turn into a falcon is excellent for escaping prisons per se - it depends a bit on the prisons' construciton, doesn't it (eg Gandalf atop Orthanc vs being thrown into a dungeon)?
When the GM negates player build choices (selecting the ability to cast falconskin) and/or PC actions (wanting to cast falconskin to escape) to achieve a particular result (inability to escape the prison), I consider that the very essence of railroading.
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"thwarting player intent" is railroading when it's dictated by the dice just as much as when it's dictated by the GM's plans for the story.
No action was negated. Nor was any build choice. The player didn't declare "I change into a falcon and escape!" - he asked "Does the prison have bars I can squeeze through?" and I answered "No." It's a moment of framing, not of action resolution - and the framing is the direct consequence of a series of failed checks (to get through the city; to persuade the guards to help with the bodies rather than treat them as cause for suspicion).
If the player had declared, in the encounter with the guards, "I change into a falcon and fly away!" this would almost certainly have succeeded. But he didn't do that, in part because he didn't want to lose track of the bodies he was carrying . . .
If a fighter's hit points have been reduced to zero by application of the action resolution mechanics, it's not railroading to deny the player of that character the opportunity to declare attacks. Mutatis mutandis in this case.
you still made the prison falcon-proof explicitly to negate the PC's ability. If I were the player, I would absolutely feel railroaded in that situation.
It seems to me there are three options.
(1) Prisons that are narrated always have bars, small gaps, etc such that shapechanging PCs can escape them (ie can never be held in prison).
(2) Prisons that are narrated never have bars, small gaps, etc.
(3) Prisons that are narrated sometimes have bars, small gaps, etc; and sometimes not. Given that my setting is a trad fantasy one where timber is more prevalent than worked metal, (3) seems the most logical.
Then, within 3, there is the question - how do we decide which prisons are which? We might roll % dice. BW has a mechanic for that (called the "die of fate") but it is not a major part of the system, because it only comes into play when other considerations that would inform framing and resolution are exhausted.
Another way to decide would be via action declaration and resolution: the player could have tried to roll an Architecture or similar check to discover the gap his falcon-form can fly through. But he chose not to - presumably because he suspected he wouldn't succeed at that.
Given that a DoF was not appropriate, and that the player didn't declare any action relevant to the construction of the prison, my framing stands. It's a consequence of failure; hence, you didn't get what you wanted (ie you're not getting where you wanted to get with your bodies); hence you can't just fly out in falcon form and negate the failure. You're going to actually have to deal with the failure. (Which he did: he created an illusion of himself and the other prisoner; turned the two of them invisible; and then the assassin tried to pick the lock, taking the time to make the attempt with care. Unfortunately that check failed, and so - per the rules of the game - I am entited to impose a major time-based complication (due to the taking of the bonus for a careful attempt). That was the cliffhanger on which we finished our short session last Sunday.)