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 tags. Add a new line under that area with:

Invulnerable
DoNotDieAtZeroHitpoints