Since version 6.5, Castle Game Engine includes an Editor. You can use the editor to design a user interface, which is just a tree of TCastleUserInterface instances. Such user interface can be saved to a file with extension .castle-user-interface and later loaded in your game using a call like UserInterfaceLoad('castle-data:/my-ui.castle-user-interface').

The editor can be used with any Castle Game Engine project containing a CastleEngineManifest.xml file. The editor has the same "concept" of a project as our command-line build tool, in fact the editor can also be used to compile and run the project.

More detailed documentation about the editor is here.

See also the movie explaining th editor: