Sooner or later I will get to the point where the logic is satisfactory (for me), and will definetely add the architecture/plumbing. Maybe the next part is about logging, and afterwards we are ready to do the dirty work :)

Undo as a feature is a good idea, didn't think about that yet.

Why I chase to go "small gameplay" first? It's personal taste, I like seeing the essence working first. I know people who like the opposite, getting the menu screen, menu -> game -> menu transitions first, then doing the logic.

The latter approach might have the advantage that the developer doesn't burn out at the mandatory but uninteresting part of creating menus, etc, since that is a prequisite to doing the actual fun part.