Released in 2015, Kingdom is a sidescrolling, pixel-styled management game about the trials and tribulations of royalty. You play as either a king or a queen, and it's your job to make sure your kingdom doesn't decline and perish. This is achieved in numerous ways—you can even toss cold hard cash at your subjects to win them over—but as James noted in his review, a zen early game quickly becomes very stressful.
The game is currently free on the Humble Store, where it would normally set you back $5 (AU$7). You've got just over six days to secure it, and once you've secured it, it'll be yours to keep forever. Redeeming the offer provides a Steam code, and it works on Mac and Linux as well as Windows.
Of course, this is the original "classic" version of Kingdom. Since then, Kingdom: New Lands has released, and it adds a whole lot more stuff to the formula. Chances are you grabbed that last year when it was free on the Epic Games Store, but there's no harm in having the game that started it all as well.
Don't go in expecting a walk in the park, though. "Kingdom is home to a beautiful, mysterious world I desperately want to explore," James wrote in his review. "There were thrilling moments where a massive army of monsters were throwing themselves against my last defenses and I barely managed to make it to morning—it just takes too long between failures to reach the point where I find opportunity to grow."