Fantasy worldbuilding in itself takes on a different style than other methods of worldbuilding. For example, Fantasy worldbuilding is nowhere near as detailed as Sci-fi Worldbuilding on certain fronts. However, there are some things you should do and some things you should avoid doing when it comes to creating fantasy worlds meant for Roleplay use. DoDon't Establish the world, basic geography, important locations and political boundaries - A fantasy world, like any other world, requires this basic skeletal structure. Without these things, you're left with pretty much nothing. Describe the planet in scientific terms such as rotational velocity, rotational period, gravity relative to earth's, et cetera - Most fantasy worlds aren't meant to be made realistic. There is a certain atmosphere when it comes to fantasy and approaching the world from a scientific POV will certainly be jarring. Establish lore - For a fantasy world, especially one that follows a "chosen one" plot line, lore is a very important part of worldbuilding. HOWEVER, for the love of god, make sure it is at the least amount of detail possible. Mostly for realism reasons, but also so that you can retcon easily. You can have the lore in a personal place written out completely, with all the details, but in saying that, you should keep it as vague as possible without detracting from the important parts. If you're as evil a schemer as I am, doing this will give you space to surprise your RPers. Make the lore perfectly accurate - Especially true for a fantasy world, certain areas of history are bound to be vague due to disaster and/or conflict. Not only this, but history is writ by the victor, and in times of great conflict, the victor is the one that survives to write history. Large swathes of your world's lore would, realistically, be unclear. Not only does it allow for easy retconning, but it also allows for a more open style of play. Creating lore that is accurate down to the punctuation marks removes any potential surprises that might come from history. Describe the major races in detail - Many Fantasy worlds have multiple races in them, some hostile, some hospitable. It is important to elaborate on these races because roleplayers will be choosing from them and it would be easier for the roleplayer if you are able to provide a good representation of the race. Describe them to extreme levels - How much is too much when it comes to describing the major races? Well, if you find yourself describing average penile length, you've probably gone too far. On a more serious note, though, too much is when you begin to restrict the creativity of your Roleplayers by describing every little detail of your race. Make sure you establish the magic system(if you have one) firmly. A magic system, especially if introduced in a fantasy world, is an especially vital part of it and while it might not be central to the plot, if you don't describe it in detail or at least have a good idea of how it works, you're going to be in for a lot of trouble. Make a vague magic system. This makes it easy for would-be godmodders and Mary Sues to exploit the system. Trust me, you don't want that. These kinds of characters/players typically ruin the RP experience for everyone. You don't have to tell the players everything about the system, but make sure that you have it really set in your mind so that you can tell someone "Hey! That's way too powerful for your character, et cetera, et cetera." Please, just, please have a good idea of how politics works in your RP. Is it a Game of Houses? Democracy? Meritocracy? Are leaders chosen by their prowess on the fields of battle? Just put in a basic framework for politics. It will help a long way when you get to the part where the RPers have audiences with political leaders. No. No. No. Not every Fantasy RP is a Political Intrigue one, and unless you want your RP to be a Political Intrigue, you've no need to do anything more than set the framework and flesh it out a little. Maybe names of prominent tribes/houses, electoral system, judicial system, nothing more. If you want to detail it out, keep it to yourself at least as the RP starts. No one needs to know that yet! You could reveal these details to the RPers as the opportunities to do so come. Try not to be too sensitive about your world. If it's something really, really important to you, then only play it with trusted friends, because roleplayers come from a whole slew of different backgrounds, walks of life and personalities. They're bound to see the world you've made in a different light than you. If you still really want to share it with other people, you should be willing to be flexible with your world based on the needs of your RPers. Be Mr.GM who goes "OH NO. THAT WON'T WORK. YOU STUPID STUPID FOOL!" It's bad for you and it's bad for the RPer. So please, just don't. Of course, there are certain things that no matter how you lawyer and bend and flex your world that just won't work. Don't hesitate to tell the RPer about that. As much as you can, try to offer an alternative that is kind of close to his/her concept and actually works in your world. Remember, there's a fine line between description and restriction. When it comes to RP, that line is very important because it will determine how easy it would be to hold your RPers' interest in your RP. Oftentimes, if you are far too restrictive and you don't give the RPers the ability to be creative with where they want to go, they will quickly lose interest in your RP. This is not the only way to go about, but it is the way that I have found, over the years, to be the most effective. Don't take this as a set of laws you have to follow to the letter, but take it as a guide. Feel free to ignore some parts, but don't ignore the spirit of the guide: be as detailed as you can without restricting your RPers.