Back in the beginning none of us saw a problem with a lone Orc guarding a treasure chest, oblivious to the sounds of combat in the chamber next door. The Orc would duly await the adventurer’s arrival and then fight to the death to protect the treasure. Things have moved on a little since those days of simple pleasures. The GM now has to come up with detailed storylines, monsters that cooperate with each other, and complex multi-level encounters.
I still like the 10'x10' room though. It’s a staple part of D&D mythology just like the Dungeon Crawl and starting an adventure in a Tavern. If I include an underground complex in my games I feel compelled to include a 10'x10' room as homage to our roleplaying origins. Maybe this is because I still think its possible to make this smallest of rooms an interesting place for an encounter.
For a start a confined space like this is a great place to put an area effect trap. A well placed fireball with everyone standing closely packed can be great fun for the GM. Gas traps, Pit falls and rock falls also work well in a confined area. And of course a 10'x10' room is the perfect size for a Gelatinous Cube dropped from above on a group of greedy adventurers.
Aside from opportunities for lethal fun small rooms can also fill a number of more mundane roles. Well shafts fit nicely into a small room like this so maybe this room houses the dungeon complexes water supply. It could also be a store room of some kind filled with books, alchemical components, weapons or food. Small chambers can easily be overlooked by other adventuring parties that decided to bypass the area for larger pickings elsewhere. This may mean hidden caches of loot.
Not everything is as it seems, especially in a D&D game. That small, insignificant room may hold a secret door to a hidden passage. It may be a treasure chamber cunningly divided by a false wall. It could in fact be an illusion designed to make a larger more important chamber look like a the sort of room you'd put a Garderobe in.
But for me, whenever I include a 10'x10' room in my dungeon complex all I want to do is put a an Orc guard in there, sitting patiently on a treasure chest and waiting for the inevitable.