When I get the time later today I'm going to upload a copy of a
shareware game I found over on the Empire of the Red Dragon board
called Cannon Fodder. It's a 2-D cannon volley game where you and an
opponent lob cannonballs at each other, trying to destroy each other's hospitals, command posts, and most importantly, cannons. There's a ton
of customization down and I find it super fun even if I can't find a
player 2 (no computer control, unfortunately), and it runs on a Mac
Plus more than comfortably. I haven't tested it on anything else, but I
imagine it runs on a 512K just fine as well.

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-nkeck72 (Mac Plus)