I'm writing a fantasy novel and while sticking to reality shouldn't be much of an issue in a world with magic, I still try to do so. With that said, I try and research as much as possible before writing about things.

From what I've gathered the steel used in swords was generally, historically, of higher quality than the steel used in armor because the methods at the time, say the 15th-16th centuries, did not allow smiths to remove as many impurities from the larger pieces as they could with swords. So a breastplate, even one from a master armorer, would contain more slag than the typical sword. Is this correct? And even though, within the novel, there exists the typical fantasy style nearly unbreakable swords, this would not guarantee the ability to create suits of plate out of the same steel because the general level of technology, again late 15th-early 16th century, simply isn't there. And I guess I might as well ask whether creating laminar armor out of the same steel would make sense in context.

It is definitely not true in the 15th-16th centuries. The steel used in Milanese and South German armour (the two major centres of armour production) could be just as good as the steel used in weapons. It is largely irrelevant since, even with poor quality steel plate, you still can't penetrate it with a sword no matter what the blade is made from or how "unbreakable" it is.

This is more a personal taste thing, but one thing that always anoys me in worlds fantasy novels /games where magic works is that it seems like it is AD 1420 (except "the North" whee it is AD 923) and technology has developed exactly the same untill last week when magic showed up. Castles and defensive structures are one spot where this shows up badly. I don't think a culture that has access to (or even knows about) magic that lets people fly (or even teleport) would have designed castles with 30 foot tall walls, since the magic renders them useless, though stilll presumably expensive.

Now, I don't know how magic works in your novel, but I find it hard to believe that in a world where magic is demonstrably effective that some enterprising armoursmith would not have aproached a wizard (or sold his soul to a demon) and said " hey, do you think you could magic out the impurities from this bloomery iron for me?" or "hey, do you have a spell that could weld the ends of these chain links together so I don't have to make all these tiny little rivets?"
History has shown that humans have a decent ability to develope technology, but a far greater ability to exploit those developments.

Thanks for the reply. I wasn't suggesting that the swords could penetrate plate. I was just trying the justify the absence of armor made from the unbreakable steel by having the process to create it be so complex that creating large pieces of it would be all but impossible. Armors of the time were good. I remember an episode of the Master about Konrad Seusenhofer, Maximilian's Master Armorer. IIRC a gift of Seusenhofer's armor to Henry VIII led to the creation of the Royal Armory in Greenwich. But I read something, and I can't remember the source, saying that armor of the time contained more impurities than would be found in the steel in swords since working large pieces of steel was difficult at the time.