3D background object like Castlevania

Unfortunately your best bet would be either playing with some kind of parallax (either layers scrolling at different speeds or parallax object) or making prerendered 3d video (avi and so on) that loops and has no sound. Otherwise, without some kind of 3d engine such as P3D or Firefly it's not possible.

//edit: P3d is now $16 on ClickStore. While not as powerful as Firefly, especially when it comes to rendering, it should be enough to get the effect you want.

if youre talking about the moving clouds whooshing toward the viewpoint, you can do that without 3D, by using scaling and skewing of the sprites with interpolation.

That's how they did it on the original 16-but game machines.

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oh the tower? yeah youre going to want a true 3D solution for that I think, especially if it is angling based on the player's position in the world.

If it only matters the player's PROGRESS through the game or the level and not their vertical position though, then I wonder if you could make it a long animation or video flyaround of the castle and select which frame of the animation or video it is (like scrubbing) based on progress forward or backward through the game

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What makes you think the tower is 3D? From what I saw in the video it doesn't change perspective at all. To me it just looks like 2D sprite.

If you're just intending to use the effect to add a little eye candy here and there (as far as I can tell, that clock tower is the only time the effect is used in the entire video), then Firefly and P3D would both be serious overkill.
Your best bet would undoubtedly be to buy P3D, but instead of actually using it for your game, just take one of the shaders from it, and work directly with that. Don't get me wrong - P3D is great - but it's a powerful and complex 3D engine, and would add hundreds of events to your game, whereas the very simple effect you need could be achieved in only one or two lines.

Agree with Muddymole, it's maybe a bit too oversized solution for the task,
you can think having a "synced" (with player movement) animation, or probably a set of different parallaxed sprites to achieve that effect (I think the camera only moves "sideways" so you don't need total 3D for that).

Also wanted to specify you'd have probably difficult times if you wanted to interact just with the shaders in version 2.0, as they are not very readable and need a very specific set of data to be thrown them (three vertices, plane normal components, textures ratios, zoom ratio etc.) since they were specifically designed over P3D.
I don't think that would be very viable either, unless you use previous versions of P3D 1.x, taking advantage of the great Quad shader (kindly made and donated to the world by Mr. Muddymole here :)) which was indeed very useable and readable because tailored for a different set of tasks.

Anyway, I had fun making a tower clock example and I thought I'd be posting this here too,
in case anyone reading the post for other means was interested in checking it out:
Please login to see this link.

(And Jonathan I'm sure you'll have fun with P3D anyway XD thanks for supporting it!)

Wow, that is awesome.