Note: This procedure involves editing a game file; create a backup copy of the 
file before proceeding. Use a text editor to edit the "proto.xml" file in the 
"data" directory in the game folder. Find a the heading for a unit in the file 
(for example, "Explorer"). Locate the area below the heading that starts with 
the [Flag] tags. Add a new line under that area with: 
[Flag]Invulnerable[/Flag]
[Flag]DoNotDieAtZeroHitpoints[/Flag]