A Pointless Example
Okay, let's look at the alternatives for a minute if you're not entirely convinced. I can see that some of you are determined to be difficult. Let's take your characters out of the caves, or the catacombs, or the vast space hulk, and put them outside. Wide open spaces. Nothing likely to drop out of the sky and land on your heads. Outside of the planned encounters at specific locations, what happens? Not a lot, really. Everything's miles off. It's all to easy to stroll past the encounter areas and spend hours with nothing happening.
What's that? I'm making a crass generalisation that doesn't take into account the wide variety of terrain and encounters that can only really take place outside? That for that very reason, the dungeon, constrictive and confining by nature, is simply too limited to capture the full scope of a truly effective fantasy world? One cannot live on bread alone? I hate to say this, internal dialogue, but you are talking out of your rhetorical arse.
To illustrate this, let's take some famous dungeons from popular fiction and show how, were it not for the presence of the walls, ceilings and oppressive darkness.
To begin with, we join Gandalf and the rest of the Fellowship of the Ring in the newly renamed Wide Open Spaces of Moria.
You cannot pass,he said. The orcs stood still, and a dead silence fell.I am a servant of the Secret Fire, wielder of the flame of Anor. You cannot pass.
Actually, old chap,said the Balrog, rubbing its chin,I could just walk around. Or fly if I've got wings. Have I got wings?
I dunno,Gandalf admitted.Opinion's still divided on that one.
What, even after the film?
'Fraid so,noted Gandalf.Not canonical, you see.
So I'd have to walk around, then.
Yeah, probably.
Right, then. Don't mind me; just killing your mates.The Balrog strolled over to the fleeing Fellowship of the Ring, and chopped and hacked them into bite-sized and ready-barbecued adventurer bites.
Ohhh, Bollocks,swore Gandalf.
No dungeon, no chasm, no narrow, crumbling bridge... no point. Absolute cack. Imagine Sam and Frodo's fight with Shelob taking place in broad daylight, without a veritable maze of tunnels in which they could get lost! All right, the open battlefield's full of danger too, but the hazard is all the more personal when your character is worrying about what the dungeon's denizens might do rather than being struck down by a stray arrow or suchlike.