Poul Anderson wrote:
... every kind of writing is prone to special faults. For example, while no one expects heroic fantasy (hf) to be of ultimate psychological profundity, it is often simple to the point of being simplistic. This is not necessary, as such fine practitioners as de Camp, Leiber, and Tolkien have proven.
Worse, because it is still more obvious and still less excusable, is a frequent lack of elementary knowledge or plain common sense on the part of an author. A small minority of hf stories are set in real historical milieus, where the facts provide a degree of control — though howling errors remain all too easy to make. Most members of the genre, however, take place in an imaginary world. It may be a pre-glacial civilization like Howard's, an altered time-line like Kurtz's, another planet like Eddison's, a remote future like Vance's, a completely invented universe like Dunsany's, or what have you; the point is, nobody pretends this is aught but a Never-Never Land, wherein the author is free to arrange geography, history, theology, and the laws of nature to suit himself. Given that freedom, far too many writers nowadays have supposed that anything whatsoever goes, that practical day-to-day details are of no importance and hence they, the writers, have no homework to do before they start spinning their yarns.
Not so! The consequence of making that assumption is, inevitably, a sleazy product. It may be bought by an editor hard up for material, but it will carry none of the conviction, the illusion of reality, which helps make the work of the people mentioned above, and other good writers, memorable. At best, it will drop into oblivion; at worst, it will stand as an awful example. If our field becomes swamped with this kind of garbage, readers are going to go elsewhere for entertainment and there will be no more hf.
Beneath the magic, derring-do, and other glamour, an imaginary world has to work right. In particular, a pre-industrial society, which is what virtually all hf uses for a setting, differs from ours today in countless ways. A writer need not be a walking encyclopedia to get most of these straight. A reasonable amount of research, or sometimes merely a reasonable amount of logical thinking, will do it for him. Let's consider a few points. A proper discussion would require a book, but we can make a start.