Creating levels
Levels are created using the /newlvl command. They can also be created using /os add, which has the same arguments as /newlvl except you do not specify a level name.
As usual, we should use /help newlvl to see how the command is used.
The [width] [height] and [length] arguments correspond to the X, Y, and Z dimension axes. You may press F7 in-game to see the axis lines in game.
Visualization
A word of caution about level size
If you are planning to make a level that is fully compatible with the old Java 0.30 applet (not ClassiCube), use power of two level sizes. Failing to do so will make level borders look buggy and not show up correctly. Power of two sizes include 16, 32, 64, 128, 256, 512, 1024, and so on (doubles each time).
The default theme is flat and it will be used if you don't specify a theme when generating a level.
To see all of the available themes, /help will comes in handy again: /help newlvl themes
Special note: heightmap
The heightmap theme is unique because it uses the level seed as the url of an image. This image will determine how the map generates based on the brightness. Brighter pixels = taller columns in the level.
Example usage: /newlvl imagetest [image width] 256 [image height] heightmap [url]
If the level was created using /os add, it is automatically protected and only the creator can build in it by default.
If the level was created using /newlvl, anyone can build in it by default. If you would like to change who is allowed to build in the level, use /help perbuild.
See the Level permissions page for more information.