The Campaign: When The Dungeon Is The Wall Between Two Goods

(Following from here and here.)

In Episode 1 of the 1992 OVA version of Record of Lodoss War, the party has to travel through a ruined undermountain roadway to pass swiftly from one area to another in pursuit of another objective. It doesn't go well. (See above.)

Before that, we have the Mines of Moria, which is also a route through the mountains that the Fellowship pass through before the end of that volume.

In both cases, we have one Good and another Good separated by an Evil in the form of a ruined connection that is now a den of monsters and villains- a dungeon. Like others I've mentioned, these too can, and often do, have origins in player-created strongholds.

It Was A Good Idea Then

Even now, here in the real world, we have and have had massive projects to burrow through mountains and hills in order to shorten the routes of rail and roadways from one place and another. There is no reason for why a fantastic world, where supernatural creatures and powerful magics can replicate such feats, would not be employed to the same ends- and that is before just using the actual tools and machines the Ancient World had to do such things.

But, when empires collapse and kingdoms come into ruin, the means to keep those passageways clear can fall into disrepair. Furthermore, it is obvious that predators animal and worse will come to such places knowing that prey passes through there from time to time. The smarter, more dangerous, sort actively seek to lure prey to such disused passages to ensure a steady supply of food and treasure. (Again, see above.)

Unlike a lot of the player-created dungeons aforementioned, these are unlikely to become deliberately stocked with dangerous things or valuable treasures by their creators or their operators. That runs contrary to their purpose, which is to facilitate travel--and thus trade--between regions that otherwise are ill-connected or disconnected entirely.

At most--say that this was a literal border installation--there would be a garrison on hand to inspect goods and peoples transiting the border, and the provisions for such fall into the hands of some goblinoid tribe after its original users died off or abandoned it.

While the two examples above are literal carved-into-mountain locations, a more likely example would be a massive fortified bordertown/caravaserai in a narrow and treacherous pass. When it was constructed, and maintained thereafter, the roadway was kept clear by its garrison so transiting through it was worthwhile for merchants, pilgrims, and armies alike. It was only after the fall of its master that the pass reverted back to being a dangerous place.

A Perfect Place For Multiple Parties

Dungeons of this sort always have at least two entrances, so it is entirely possible for multiple parties to delve this sort of location because they're coming from different directions. Put something in the middle that both of them may want, preferably multiple somethings, and you're going to have a lot of fun seeing if those parties meet and if so what goes down.

These are also perfect locations for successful adventurers to turn into their own strongholds once they clear it out and bring up their own followers to barracks therein and set up operations. Those multiple parties, therefore, may have the motivation to drive out unwanted presences; locations like these are strategic in nature, so control over them allows the occupant to exert power over a wide range of territory.

One can extend this to towns and cities, as they tend to operate on (near-)identical principles behind their establishment- especially those originating as military outposts, but at that point you're pushing into mega-dungeon territory- especially with cities-as-dungeons.

The Use Case For You

The most common use case is when the Referee's making a new dungeon using Appendix A on an off-night while listening to some lo-fi beats and enjoying some pizza.

The next most common use case is when a player-character's stronghold in such a location gets wrecked, which can happen. Assuming that the character in question cannot retake the lost stronghold, its ruins can quickly become a home for monsters and villains and thus return to the campaign as a new dungeon to be cleared by other parties in turn.

Mountain abbeys in a pass, undermountain border forts, forest forts where two rivers meet (e.g. Fort Snelling in Minnesota), all this and more are easily places to set a dungeon- and given how wars go, that's likely to happen sooner than later.