"The world would be one month in diameter - so that a fleet or army or inter-player message could be dispatched to anywhere in the game in a single turn."

I had a similar idea. Since my game is real-time, rather than turn based, I decided to use time for to indicate travel. There is basically no map. In that sense, it is similar to some sci-fi star games where star travel is mostly handled by "teleporting" from one star to another. Though I use time to indicate that attacking another Kingdom cannot be done instantly.

Thank you for the suggestions about Show HN. Good idea. I'll do a Show HN when it is ready.