The Bottled Setting

I may have never played an rpg with one, but I've long seen the appeal of the "bottled setting": a locale that could be the size of a small city or as big as solar system (or more) but is in some way cut off from the outside. It might also be "managed" in some way, having traits established by who or whatever did the bottling, Kandor from the Superman mythos is probably the most famous of such settings, but it shows up in rpg settings like Empire of the Petal Throne and Metamorphosis Alpha in addition to numerous places in fiction.

The inhabitants of a bottled setting may or may not know they are bottled. If discovering that fact or discovering the why or how of it is the main focus of the setting, you're may well be looking at a Mystery Terrarium. Really, though, all that stuff can just be background for a setting with any other sort of focus where the boundaries just happen to be hard stops rather than the place where things get fuzzy.

What's the appeal of this sort of setting? Well, for one, it can be used to disguise the true nature of the setting. The universe might actually be science fictional, but the "bottle" marks the boundary within which you can run a traditional fantasy campaign, if you want. Crossing the boundary can then mark a major turning point in a campaign, like potentially to a whole other sort of game. 

The other thing is a that a bottle need not be impassable. Krishna in de Camp's Viagens Interplanetarias series is a sort of a bottle wherein people from a technologically advanced, spacefaring civilization can play at pseudo-Medieval Sword & Planet heroes. Portal fantasies, in general, are not necessarily bottles but could easily be (particularly the sort involving a person somehow getting sucked into an MMORPG world). That allows players to play characters much more like themselves but still get involved in fantasy action.

In the end, though, I suppose the creative constraint it applies makes for an interesting challenge and heightens the potential for player engagement with setting mysteries. Vast traditional settings are great but there's nothing like having the players hit a wall they didn't expect to be there or have hints dropped that things aren't what they seem to get them engaged.