I can't believe I'm asking this.

ESC, Ctrl-X, Alt-X, Q, none of the old-school standards work, and there's no "quit" option that I can see. How do I exit the game without force-closing DOSbox?

It occurred to me after I posted that maybe this game never had a quit function. There were a handful of old games that lacked a quit option. You simply had to reboot. This would've been near the end of that era, but maybe that's what it is.

Oh well.

To quit, hit Ctrl-Esc.

Well done, thank you!!

CTRL+C. Works only when game screen asks us for "thy lucky number". CTRL+ESC doesn't help people playing in real DOS environment, I suppose ;).

ctrl esc doesn't work on win10, just brings up the start menu

You can change this, in the Game Folder, there is an executable called TRANS.EXE, I discovered it by accident, not only does it show you all the keybindings of the game, but you can also change them, I changed mine to CTRL+Q. CTRL+X, ALT+X and Alt+F4 are also good Quit Key Combinations, I also recommend changing the keybinding if you are running Populous under Windows 9x or Windows NT.

No, it works in real DOS, for me at least.