The instrumentation is one of the most important parts in defining the genre. In fantasy music, usually revolving around renaissance and medieval themes and settings, the instrumentation follows suit. One would generally classify any instrumentation used during those times in the respective area as acceptable fantasy music instrumentation. And that is true for any kind of music. What is native to a setting, and/ or what is stereotypically used for a setting or in a general area or culture is what is accepted by society as proper instrumentation. So when composing for a picture, game, or any specific theme, that will probably be your main focus, and the task at hand is to do research, and figure out what instruments generally belong to the [intended feel of the] time and setting you’re writing for. But if you’re really unsure and in a hurry, classical orchestra instrumentation is almost always if not always acceptable. So to make this process easiest, do a few things:
Note: Choosing 'incorrect' instrumentation isn't necessarily a bad thing; it leaves room for and interesting blend of styles and creates ambiguity; but the point of this guide isn't to show the beauty of ambiguity, but to aid in the process of creating a sort of genuine fantasy sound