In the last version Through the Ages: A Story of Civilization page 22 of the rules say:
VARIANTSIn this Game there is no extra color cubes to put in the middle so you would have to find a sub.
The following game variants are recommended in the official manual. Be sure that all players agree on a variant before beginning the game....
No Ganging Up: Players keep a color cube in front oftheir
civilizations, removing the cube when attacked and replacing the cube
at the start oftheir next turns. After being attacked, a player cannot be
attacked again until the cube is replaced.
• This variant reduces the punishment for having a weakmilitary.