I've been a GM of fantasy campaigns for 20+ years. I have created many worlds and run many others that were pre-created. I have been being asked about running another campaign by many friends, and recently started creating the world for it.
I generally use a top-down design method when making my worlds. I start at the world level, drop to the continent, then country, county and finally town area. I find the process to be very rewarding, but both time and detail intensive. I started the new campaign the same way, but had a gear switch a couple of days in. I decided I didn't need the whole world, as I was going to run a Mega-Dungeon.
I'm not really sure what made me decide on the Mega-Dungeon structure. I've enjoyed it for a long time, but have never successfully run it. I think I always tried to squeeze too much in and not leave room for improvisation for myself (the DM). I am still squeezing a lot into the new setting, but it is in a different way.
I have decided the "home base" city is inside the Mega-Dungeon (which still does not have a name). The threats can follow the adventurers home, if it seems necessary. Although the players will have the option to leave the dungeon environment, it is no longer necessary. This will fundamentally change the way the players look at the dungeon. It will be seen as home.
I have decided parts of the Mega-Dungeon will be outside. This is slightly askew of the traditional Mega-Dungeon, which are traditionally completely underground. These areas will be difficult to access, and will contain some powerful allies (or enemies). I have a Gold Dragon Lair at the top of the mountain, and a Giant Eagle Aerie down slope a bit. I'm also pretty sure I will add a Griffon Nest opposite the Aerie, and in direct competition for food/resources.
Some things in the dungeon will be staples of traditional fantasy campaigns. Some staples will be straight forward, and some will have twists. One twist I intend to have is "Elemental Bleeds." These are going to be areas inaccessible to the factions in the game, both friend and foe. As players close on the Bleeds, they will see magic and mundane items function differently. For example, if the players are close to the Water Bleed, they may find their canteens are always full. They may also find that the "Wall of Ice" spell filled the entire room, freezing everyone inside.
I could bore you with the traditional staples that would be seen, but a quick rundown of some archtypes is probably better. Players in my game should expect to see:
Orcish Clans fighting outsiders and each other
Undead being controlled by stronger undead
Dwarves hate Orcs
Humans are corruptable
Strange creatures worshiping forgotten gods
There will be almost every staple of a dungeon crawl somewhere in the Mega-Dungeon. Hopefully I can craft them well enough to make them memorable and not trite.
That will wrap up this first blog entry. I will begin working on the next one soon. In it I plan to go over the over-arching themes of the early levels of my dungeon.