Fire for Effect

The enemy endlessly bombards the cities in their way as they continue to push forward. You cannot stop their advance. But with your gun and the few defenses remaining you can delay the inevitable for a few more hours, and give the last few remaining survivors a chance to escape.
You play as a survivor wielding a detached heavy artillery piece. This is your only weapon, but its shells can knock the enemy's shells into each other, potentially causing a chain reaction. If carefully aimed it can wipe out hordes of enemies with a shot.
But the enemy's bombardment is overwhelming and endless, yet your ammunition and defenses are not. Sooner or later choices will have to be made. Whether you prioritize yourself who holds the sole weapon against the threat, or the structures which protect you but can be expendable, or the insignificant individual wounded who you need to go out of your way to save, is up to you.