These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Designated Hero and Designated Villain: The film treats Albany as a hero and John as a villain. Although modern audiences tend to oppose the idea of a divinely-appointed king, by the standard of the times, one can hardly see King John as in the wrong. For example:
The opening scene has John being forced at swordpoint to sign the Magna Carta. Signing a document under duress and then deciding not to follow it is not breaking your promise.
Under the laws at the time, King John was the legitimate king by right. This was backed by the Pope, who was seen as the highest religious authority. Albany and his men were taking up arms in rebellion against their lawful sovereign.
Albany repeatedly claims that the castle belongs to "the people". No it didn't. By law the castle and its lands were the property of the King, to do with as he pleased. Albany had no claim to it, nor did anybody else.
Albany also claims that he is fighting for "the people". If by "the people" he meant the tiny percentage of nobles and freedmen who stood to benefit from Magna Carta, then yes. But the idea of a medieval King deriving his powers from the consent of the masses was absurd. It took centuries before the idea of "consent of the governed" came to apply to anybody but the nobility.
Let's not get carried away here. The Magna Carta was primarily a document designed to cover the collective asses of the hereditary elite against that of the crown and its' centralized authority and it wasn't done by the most upstanding of methods. However, it did indeed provide a damn near unprecedented charter of freedoms- some of which did indeed encompass the entire population rather than just the Barons- that would be the building block of all that would come afterwards. In addition, John legitimately was a terrible and tyrannical leader who generally brought England misery (even though the film goes a bit too far into that direction) and at least had lost a lot of the moral justification even by the standards of the time for ruling. Secondly, regardless of what the Pope and other officials thought, there was precedent for an unjust or (more likely) just plain incompetent king being dethroned and his rule rendered null and void; John was both and while unsurprisingly it tended to get downplayed given the threat it posed to the political dogmas of the time it can easily be seen as acting as an equal or greater justification compared to the legal right of the King.
Isabel can also come across a Designated Hero. She is a Medieval noblewoman trapped in a loveless political marriage which does make her somewhat sympathetic. However the film almost at once undercuts this by establishing that her much older husband finds the marriage at least as emotional taxing as she does and he isn't interested in having sex with her (which she moans about, despite disliking him), meaning her supposedly intolerable position basically amounts to living in a comfortable castle with servants. When the Chaste Hero shows up she constantly hits on him, uncaring that he is going through a crisis of faith and acting petulant when he (initially) rejects her. Basically she comes across as a selfish Jerkass who is only interested in the hero at all because she finds him hot and wants to have sex.
At the same time, Isabel urges Marshal to question the less noble aspects of his vows, such as the fact that he essentially spends his life killing people because the Church requires him to. One can read her offer of a sexual (and emotional) relationship as essentially symbolic of a return to normal, or at least less violent, life.