Hat tip to Zopharís Domainís Message Board and Inverse for pointing out a new level editor. Quote from the authorís post on ZMD:

Hi everyone. About 10 years ago I used to play this game, and there was one thing bothering me - I always wanted to get into those rooms that have no entrances . Recently I found out that the gameís now freeware so I got the ROM and enjoyed it again, still interested in those closed rooms. Iíve been searching for a level editor or even just a viewer but couldnít find any, so I decided to make one myself. Initially I didnít plan to make any public releases but thought maybe someone has also been looking for such an editor, and here it is Doesnít have all of the planned features yet, so this is not a final release. For now it has viewing and basic editing features. You can get it here (didnít have any site so I had to create this one to host the editor, nothing elseís there for now). Some editing instructions are in the readme file included in the archive. Please pm or email me for any feature requests, questions or in case you found any glitches. Hope this will be useful for someone

P.S. The editor only supports ROMs in .BIN format so you need to use an SMD to BIN converter. I might add support for SMD in future releases, if requested.[