As the game progresses the amount of cities the player own as well as the amount of enemies increases. That is why you will always need to protect your cities from the enemy. The best way to do it is by quickly training army. Thanks to this skill the time needed to train ALL unit types is reduced. This skill is vital for all rulers, both when recovering from war and then preparing for future attacks.
How the bonus actually works:
If at a certain stage of the game a certain unit is trained for 10 minutes and you develop Overseer to Advanced, then the unit will be trained 20% faster. The time needed to train it will become 8 minutes.
NOTE: The same applies for all unit types including special units.
- Apprentice: 10%
- Advanced: 20%
- Master: 30%