Spent a bit more time pottering with yon jam entry.
I now have a pretty good vertical slice of a game:
Start screen -> playing screen (with score and health) -> Game over screen
While all the parts are relatively basic, it means that even if I were to release now, there's kind of a game in there (albeit not a very good one). A minimum viable product if you will.
Now it's a case of adding features, and polishing what's currently there.
Applying Agile software principles to game development, oh yes.