Dungeon as Milieu

"Just beyond the border lies an old castle, once a stronghold of the evil empire. The dark lord was defeated long ago and the castle abandoned in ruins, but legends say there were dungeons beneath the evil castle. Everyone knows wealth poured into that castle for decades before it was destroyed. Who knows what may still lie at the bottom of those dungeons, just waiting to be found. Now an expedition of stout warriors, and maybe a caster or two, might venture in and come away rich. Or end up dead!"
The words above could be spoken by most any referee beginning a dungeon based adventure. It could also have been from a never written follow-up novel to the Lord of the Rings. The white box has been described as a game written for dungeon adventure. While I think it is much more than simply that, the white box was certainly written with that in mind and works quite well for dungeon adventure type games.
World creation can seem a huge undertaking for a referee, therefore starting out a campaign with a single dungeon may seem like a good alternative. Draw some tunnels and rooms, fill with a selection of monsters and traps, add some treasure all taken from the white box rules and you are ready to run. Simple and manageable and for years that's the way my friends and I played the game, hand-waving any "in-town" activities such as equipment purchases, healing, etc. That was treated as book-keeping and rarely part of "roleplaying".
Unless the referee and players specifically address the cleric and deities this dungeon approach to play can leave the cleric in religion limbo. Which, by-the-way is exactly how we played white box back then. No deities, except bad ones worshiped in the dungeon. How you played your cleric was up to each player with respect to deity (or not) and any religious practices (if any). Adding a near-by town with a chapel, a friendly priest (who can resurrect?), an inn for rest and recuperation, picking up rumors and recruiting, and a store that sells most everything on the equipment list is the beginning of world-building.
Realistically, that's about all a referee needs for a fairly good campaign. Gaming in this manner makes the dungeon the centerpiece of play and it is the dungeon that defines the milieu.To support an interesting game the dungeon has to be more than a collection of rooms, monsters and treasure. It should be a memorable "place". Something should set it apart from other dungeons so that the players recognize it and recall they have been someplace worth visiting. Give it a name, some character. Make use of special places within the dungeon, architecture, history, inhabitants and treasure which all have a uniqueness to them.
"Thousands of years ago there was an ancient civilization here. They buried their mummified kings with gold and gems so that they would be wealthy in the afterlife. To protect these royal tombs they designed elaborate structures with hidden entrances and other defenses to detour would-be robbers."