Bukkit

'Dragons Lair' is build around the idea of having an open world RPG that can be played more than once and by more than one person, but still let the one person be able to save his current state and continue on that whenever he wants. To be able to save it and let other people play it as well, you need a so called 'dungeon'. Because it's open world, you don't have borders defining the dimension of your dungeon - you can make some yourself though.

If you want to start a dungeon, you need to register first which gets done by an event. Until the dungeon gets started - again using an event - as much people as you'd like can register to play inside the dungeon. Those people play together in the dungeon trying to find a way out. You can, if you want to, make a maximum amount of players per time as well as a minimum amount. Everything that happens inside the dungeon, e.g. breaking of a block, gets reverted when the party decides to stop or pause (-> save it's current state) the dungeon. If they paused it, they can register, start the dungeon again and they'll start where they'd left off. You can pause the dungeon as often as you want and it gets saved even through server restarts.

As well as in a rpg you have chapters, which represent different parts of your story. Also, each chapter has objectives, which tell the players what they have to do and they can decide on how to do it. You can use the same chapter or objective in more than one dungeon without creating one for each dungeon separately.