Man, the costume porn in The Thief of Bagdad (yup, still the 1940 version) is off the scale. Obvy, there are a lot of fantasy elements, but they are vastly outnumbered by the sort of historical detail that's un-fucking-believable.
They actually went to the trouble of embroidering tiraz bands (gold or silver embroidered strips with Arabic prayers to the emirs and caliphs or with well-wishes for the wearer) on some of the robes, which you never see in movies set in this era. Because, to be honest, where the hell can you get embroidery like that without having to spend bucketloads of time and money, and when it's easier to just slap something generically sparkly on? The bumbling old sultan has some tiraz bands on his robes, and I strongly suspect the costume people just made them from scratch. That's dedication right there.
Note that in the background, you also get a highly accurate depiction of an Abbasid palace guard (although maybe he should be wearing boots instead of sandals, if he's one of the elite troops who had to do quite a bit of riding around), but still, that's pretty much spot on.
Oh, and in an earlier scene, the sultan's wearing green (a colour reserved for royalty and/or whatever ruler who claimed descent from Muhammad) and the circular designs on his robes are so accurate it makes me want to scream in joy. Although you could be forgiven for thinking I'm actually screaming and tearing at my hair over Jaffar being such a sexy motherfucker. Whose robes should maybe be a bit blacker, but there's still a glorious amount of black for someone who's a high-ranking Abbasid vizier, since that was the official colour of politicians and civil servants of the time. Okay, and of Evil Grand Viziers Who Are A Bit Wizardly And Shit And Called Jaffar, but whatevz. And don't even get me started on the drape of the turban or the way his dagger is tucked into his belt because I'm pretty much close to humping the screen right now.
Oh, and you know, chess was big in the 1001 Nights' era Persia before it had even made it to Europe.
ETA: Here's a better picture of the guards' costumes. The swords tucked into their belts were the most common way swords were worn at the time, although I should point out curved sabres (so often associated with assassins and Saracens of the Crusader era) only became popular a bit later in the Middle Ages (from about the 1100s onwards). Note that I say "popular", as there are 9th century examples of curved sabres being owned by posh people, so it's very likely Jaffar would've had one. Note that he's wearing it hanging from his belt, also a later and much more decorative style, which probably denotes poshness and decadence and that his weapons lie elsewhere--in his brains and his black magic skillz and his motherfucking sass. Click for bigger.
Every single extra (and there were a lot of them) could be placed in a specific class, locale or occupation or even mood on the basis of his/her dress, and the amount of detail was just staggering. Why the hell would you need to include a pet cheetah? Because fuck you this is medieval Persia, that's why. Oh, and the cheetah trainer guy's costume is based on somewhat later Indian ones, but probably didn't change that much in the whatever 1000-odd years they used cheetahs for hunting gazelles. Aaand I'm pretty sure I remember that tricolour thing the African dudes are wearing, too. Although if they're meant to be slaves, it would be inaccurate, as I think that's something Ethiopian mercenaries wore. Or what the hell do I know, maybe they are badass ex-mercenaries who have retired to cheetahkeeping or something.
The regular viewer wouldn't even know that a girl's actually dressed as a boy for plot reasons. I mean, would you think this outfit was particularily manly? If you weren't a costume nerd or Central/South Asian, the answer would probably be "no". The only things that distinguish this costume as that of a young boy's are the turban (women would have worn veils or caps--turbans showed up sometimes, but they were rare on women before Ottoman times) and that ribbon the girls are tying around her lower chest. That ribbon is totally and utterly unnecessary, but they just had to put it there because bar the turban, it's the only thing that says "guy" there. If you are familiar with Persian miniatures. These days, if a movie needed to denote stuff like that, you bet they would've given her something very Western-butch to hammer it home. I'm so glad they didn't, here, because it's such a bonus for the history buff.
Oh, and when Jaffar shows up to woo the princess, he's dressed in pure white to signal his (possibly false) pure intentions, and that's probably something the regular viewer will get. But did you know he's also dressed as a bridegroom? I bet you didn't. Take me, evil vizier lord.
And I'll leave you with Mary Morris, who was so young and hot I totally didn't recognise her at first. Particularily as the first thing I saw her in was Kinda, where she was as gnarly as an old tree. Oh, and she's Jaffar's henchwoman. Because, let's face it, every evil grand vizier needs a lesbian henchwoman.
Aaaand unless I'm totally mistaken, that high hat of hers denotes a Northern Persian style, possibly descended from the old high Scythian hats and I'll just shut up now.