I don't tend to do design docs before starting a project as I like to just dive in and let them evolve as I progress. It makes coding a little interesting to say the least but thats half the fun.
I am getting slightly better at keeping notes now as it's essential when working as part of a team, but this still amounts to a few notes, feature ideas and code snippets in a notepad file which I delete as I add them to the program. When the file's empty the project's finished

Anyway each project still starts the same way. Fire up Paint Shop Pro and do a single concept screen of what the game might look like and of what elements I'd like to include.
Here for your amusement is the DUO concept sketch (dated 5/10/2003) . I'd like to say it's rough but in reality it represents the limit of my graphical skills and, worryingly, doesn't look a million miles away from the finished game.....well it's recognisable anyway

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