Traditionally a combination of border types are used, for example any combination of:

• Water
• Cliffs (fall to your death)
• Mountains
• Lava
• Forcefields (better than an invisible wall)
• Impassable forest
• Fences/walls/buildings
• Instantly fatal opponents
• Game-specific arbitrary limits (e.g. collar blows your head off, game ends because you abandoned your quest, etc)
• Limited resources (e.g. theoretically unbounded but you run out of air/die of radiation poisoning/etc)
• Boringworld (can walk forever, nothing interesting will ever happen)

Islands are fine, but I personally like a combo both for variation sake and because I secretly believe I will find some way through. ;)

You forgot mine fields and angry animals that will eat you! :D

Flying island (my favourite :))

I like the way they did it in the movie dark city. The city didn't have any exits or streets leading out it was just a large circle with a wall of buildings.

I like the way the they didn't it in the movie dark city. The city didn't have any exits or streets leading out it was just a large circle with a wall of buildings.

That was interesting. Inception was rather interesting also in that regard. A dream city where buildings and roads are moving around and unfolding as you explore, and steering and guiding you in the direction the game wants to take you.

Now that would be interesting. It would take some doing but having the all paths lead you to the same destination would make an interesting game mechanic.With the city shifting and changing so you always arrive where the game wants you to be.

You could also tie some vital resource to a central location and make it diminish the farther the player gets from it: For instance, a mana stone that causes the player's spells to be weaker the farther they are away from it, or a microwave emitter that gives vehicles and gear less and less power beyond some range boundary.

You could also wrap the world if it's magical or supernatural, so that leaving east returns you to the west side of the map ala old 2d games.