From Ardrana

By means of the armor spell, the wizard creates a magical field of force that serves as if it were scale mail armor. The spell has no effect on a person already armored or a creature whose natural protection is better than scale mail. It is not cumulative with the shield spell, but neither does it hinder movement or prevent spellcasting, and adds no weight. It lasts until successfully dispelled or until the wearer sustains cumulative damage of a modest amount, depending on the spellcaster's power.

Note: This version of armor originally appeared in the 2nd Edition Player's Handbook for Advanced Dungeons & Dragons from TSR. Its use here is for the purposes of providing context for the campaign only.