0 A.D. is a free, open-source, cross-platform real-time strategy (RTS) game of ancient warfare. It's a historically-based war/economy game that allows players to relive or rewrite the history of twelve ancient civilizations, from Iberia to Mauryan India, each depicted at their peak of economic growth and military prowess. Developed using Pyrogenesis, a ground-breaking new game engine custom-built to suit this project, 0 A.D. will give players a rich and entertaining real-time gaming experience.
"Exponential Armor" is the exciting new armor paradigm in the game. Each additional level of armor reduces damage taken by 10%.
This means, if you have an armor tech (e.g., "Silver Shields") that adds +1 pierce armor to all units, it will reduce the pierce damage taken by all units by 10%, regardless of how much pierce armor each unit started with.
This helps with balancing and ensures a tech doesn't affect different units differently, unless intended to do so.
It's also just cooler to say "+1 Armor Level." ;)