Magic is the key to the fantasy genre, the most important thing. This is not lost on most authors, but many do not know where to begin—I know I took a long time to develop my world's system to the form it now holds. However, I had a good idea of what I needed it to do, what I needed it to be.
First, a note on medieval fantasy—MAGIC CHANGES THINGS. Obviously, but exactly what things it changes are sometimes less clear, so here are some examples:
1) A castle wall will not do much against a dragon, griffin, or other large flying mount or threat. If air power is common, then defenses have been developed that are capable of dealing with it; that is simply the way people are, think, and work. These will likely take the form of air-to-air combatants (dragon riders or whatever that rise from the walls to attack the enemy) or, for want of a better term, anti-aircraft guns (one possibility is known as a ballista; basically start with a crossbow, and then expand it to the scale of firing six-foot-long logs—I am reasonably sure this was a real, historical weapon) or, most likely, some amount of both. On Earth, both of these became very apparent on the battlefields of World War One.
2) If teleportation or air transport in any form is readily available, then most people will not walk. Most people on Earth that plan on going a long way take a plane, and the ones that do not have a specific reason. Examples include airport security and sightseeing en route to and/or from the destination.
3) If wizards are common, then body armor has been developed that can protect against most of their spells. In other words, if magic or magical weapons can pierce a knight's armor, then his armor has been modified to make this no longer the case, unless the spell in question is recently developed or recently revived from records. Warriors are not sent into battle vulnerable, because people are not stupid. Earth soldiers have Kevlar vests and flak jackets, for exactly this reason. The "magical weapon" in this case is gunpowder (remember how the American natives first reacted to European guns—"they were magic! They were sorcerers!").
These examples are simple, quick, and possibly exaggerated or oversimplified in some cases, but I believe that they make my point. A purely medieval world will not work when magic in any form is introduced. It will react, it will change. The following chapters are intended to give more specific advice on exactly how it will change.
Chapters in the Guide—along with a rough explanation of what I am trying to do with them.
1. Introduction—explain why the effects of magic on society must always be considered by a world builder.
2. The Five Questions—A short list of questions to consider, along with an explanation segment where the importance of each is explained.
3. The Many Questions—A much longer and more detailed list of questions to consider. Having answers for all of them should give a reasonably well-rounded picture of magic in a prospective fantasy writer's world.
4. Magic of Earth—Answers to the questions of chapter three for Earth "magic"—the technologies of chemistry, electricity, and nuclear-based applications. Also meant as food for thought.
5. Magic of Selvaris—(Coming Soon) Selvaris is the fantasy world in which I have set my stories so far. As such, the answers here may be somewhat more pertinent for a prospective or current fantasy writer. DO NOT PLAGIARIZE.
6. Magic, Technology, and Science—(Coming Soon) These three things all interact and interrelate. Here are some hints and advice on how.