Page "Civilization III" Paragraph 28

Each unit has an attack and defense value that is compared against another unit's appropriate value ( attack vs defense ) to determine the winner of each battle.

Certain terrain types, as well as large cities, defending across a river, and fortifying a unit provide additional defensive bonuses ( e. g. a mountain has a 100 % defensive bonus, so a unit with 3 defense will be considered to have 6 defense when defending on a mountain ).

Ultimately, a random number generator ( RNG ) determines the outcome of each battle, so it is therefore possible ( although rare ) for a Bronze Age Spearman to defeat a Modern Age Modern Armor.

This issue was supposedly dealt with in Civilization II with the addition of firepower and hit points.