Easy! Every noob could have done that.
Decoding the level files and writing the editor for it was much more hard work.
Hard to get the stuff working under windows, especially the creation of a fully working ROM
to test the level directly within an emulator. Cool eh? For the x-mas/new year video I had to
do the level with an hex editor. Now everybody can do that easily.
How to create a ROM with more than one level? Actually, I have not foreseen this in the editor because you need some tools which
are not working on windows newer than 95 or 98 or so. And scripting with editors is not easily done. BUt yes. It is working for me.
Can I add custom sprite sets? Yes! I have foreseen five more spritesets. But praktically you cannot change the objects part (animations, sounds). Getting the color palette correctly, is another big problem. If you get that right, let me know, I build the ROM for you :-).