This review contains unmarked spoilers for Fullmetal Alchemist 2003.
It does not spoil Conqueror of Shamballa itself.
Review in brief:
Conqueror of Shamballa attempts to be its own thing when it's a sequel to an anime with loose ends and completely fails to succeed as either. It's almost as though this movie was initially a failed standalone pitch before being shoehorned into FMA as the 2003 anime was reaching its end. As disappointing as FMA '03's ending was it's far better than anything that Conqueror of Shamballa has to offer.
Review in full:
Anyone who finished FMA '03 knows that its ending was just a setup for a sequel. Aside from the implicative final scene there were still a number of lingering questions regarding specific details from the show that were left unanswered. This movie serves to end the anime-original storyline, but nothing more. It has no desire to answer the questions the series left open and instead seeks to stand on its own as the final arc of the story. This in itself raises serious questions about the validity of its setup (FMA '03's finale) but considering how much of the series' rules this movie forgets they may as well have just forgotten about what they had done before. This is especially true when it comes to "the gate", the means by which alchemy acquired its energy to function in FMA '03. The TV show went to great lengths to show that transmutations had a cost (except the last one that got Edward & Envy to Earth, apparently "the gate" stopped caring). Now "the gate" arbitrarily lets people switch worlds (either freely or at a cost) so long as they can open it. The old rules simply don't apply anymore, especially when you consider that a certain event at the movie's end involving "the gate" would render alchemy impossible according to the TV show, but the movie doesn't even consider this.
Even within its own story, this movie features an unending torrent of plot holes & conveniences. Many of these revolve around characters (and most of the characters at that) doing things not because it fits their presented character or because it's a sensible thing to do but simply because otherwise the plot would not have moved forward. This wouldn't bother me as much if it wasn't the excuse for such events as Edward cheerfully deciding to miss the public unveiling of his past 2 years of work for a quick snooze because it was apparently the only simple way to set up the next scene, or Fritz Lang crashing the Beer Hall Putsch because he magically knew Edward would be there, or just about everything Hohenheim does. The whole movie is like this too, in fact it's difficult to give examples without giving spoilers. There's more hole than plot and it's not even funny.
By "crash the Beer Hall Putsch", I do mean Fritz crashed his 1920's automobile into the beer hall where the putsch was just kicking off.
Nearly all of the characters (including the new main characters) are cardboard cutouts who are either only present to push the plot another step or enforce a stereotype because this movie likes to think it has a central message (it very well could have if it had actual characterization and could remember anything it said for more than 10 minutes but that is certainly not the case). The few characters with any meaning to them derive all of it from the TV show, so while Edward has clearly matured since his state alchemist days and Alphonse (Elric, that is) has become bolder (and can fragment his soul to control objects now, no reason given as usual), none of the development is actually shown and rather is just what we're given from the get-go, which means that most of what they do is just set up to push the plot forward as well. Meanwhile you have characters like the main villain who attacks the people she was supposed to get help from (because how else is someone who leads the antagonists supposed to look evil?) and Maes Hughes who has been reduced to a laughably shallow evil nazi stereotype.
It's frustrating considering how little the setting has been tapped into by general media. Weimar Republic Germany was a time of wild uncertainty with revolts and movements of just about every kind that would have been possible during the Interwar period. Many of the important events and people associated with World War II were already set in place during this time, and Germany was at the center of it all. Conqueror of Shamballa does tap into its setting (though events revolving around "the gate" and the antagonist Thule Society are the main focus) and provides an interesting (though hard to believe, mostly due to poor execution) theory regarding the Beer Hall Putsch. The problem is that it has nothing to do with Fullmetal Alchemist and would have likely worked much better as a fully separate entity. What few elements that were brought over from FMA almost all appear shoehorned in to remind the viewer that this is in fact a FMA sequel and certainly not something completely different. FMA's world could have been swapped out with any other fantasy world without any other change.
Conqueror of Shamballa's graphics are quite standard for GameCube titles of its time.
Finally, this also isn't a movie that features worthwhile production values. The sound is merely alright with passable music, forgettable effects, and (in Japanese anyhow) voice-work that only could have gone as far as the flat characters who were speaking to begin with. While the animation quality is generally slightly better than the TV show it also features highly dated CGI and plenty of stilted moments. There's definitely worse but there's something to be said when a 2005 movie can't truly improve over a 2003-04 multi-cour TV series.
It's the FMA '03 finale and it has so little to do with FMA that it's painful. If you want a sequel that cleanly ends FMA '03 rest unassured that there are no answers, just holes. If you want a good standalone movie set in the Weimar Republic then I regret to inform you that you won't find it here. Those are the only two things this movie attempted to be. You can skip this movie without missing anything worthwhile, in fact I wholeheartedly recommend avoiding it.