This is the key problem, unless you want an incredibly generic, bland, one-size-fits-all environment. Unless you were to make it the Disney Theme Park version of the Middle Ages, a female character would face:
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The Holy Roman Empire had VERY strictly defined gender roles in society. While England and, perhaps, France were to varying degrees more egalitarian (and even then bear in mind we’re only talking by comparison to the HRE), with both England and France allowing women to be appointed to the various orders of Chivalry, the HRE was much more restrictive. In fact it was outright ILLEGAL for a respectable woman to visit a tavern without a male escort, usually her husband (in other words, a woman by herself at a tavern would likely be assumed to be a prostitute). It goes both ways, of course. Brewing was considered a woman’s profession during this time, so outside of monasteries it would be extraordinarily rare to see a male brewer.
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A woman ignoring these gender roles would face SIGNIFICANT hostility. And not just from men, but from other women, as well. While in our 21st century culture we (mostly) believe that anyone can be who and what they want, regardless of their gender or income level, this was NOT the case in Medieval Europe. Who and what you could be were strictly defined by both your social class and gender, and someone who ignored that was very literally upsetting the “natural order” of society. People view that as threatening and dangerous, and they WILL respond negatively, if not violently.
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Wrath of the Church. While much of what happened to Joan d’Arc was politically-motivated, and even AT THE TIME it was outright acknowledged to be a miscarriage of justice (so that’s saying a lot when you consider the justice standards of the day), the Church itself enforced those gender restrictions. A woman dressing like a man – specifically, wearing PANTS – actually WAS considered heresy. To say nothing of a woman going around doing “men’s work.” So yeah, the Church wouldn’t exactly look too kindly on “Henrietta.” Especially because unlike the average townsperson who might stop at hurling garbage and insults, the Church has the actual LEGAL authority for far harsher measures…
Of course, the Church DID have a loophole: Women WERE permitted to dress in men’s fashion in appropriate situations. So a woman that does a lot of traveling (like your generic fantasy sword-swinging adventuress) might be given dispensation for pants, because wearing a dress might be impractical on the road (using d’Arc as an example, she was allowed to dress in men’s fashion while imprisoned because the guards were raping her when she was wearing dresses, and pants offered her better protection). But that mostly means that while a female could still be prosecuted for violating social conventions by serving in the army, the Church just wouldn’t add charges for wearing pants on top of it.
In other words, while it’s not IMPOSSIBLE for a female character to be made to work in this story, for it to be in ANY way realistic would basically require an entirely different set of character interactions (for that matter, she might need TWO sets of interactions if you want to have different reactions depending on how a female approaches another character. IE, dressing in men’s fashion might provoke hostility, while dresses may yield a more positive reaction).
COUGH
http://www.bowlore.com/0.02_0.04_0.02_0_308_409_csupload_49612398.jpg
Depiction of a woman from the I.33 manuscript. I’m not sure if she’s intended to be the master or the student on these folios (possibly the latter, as the teacher is almost universally rendered as a priest or monk), but regardless it’s a VERY clear indication that women of at least some social classes DID receive formal training at arms.