"Protective" is one of the leader traits, introduced in the Warlords and Beyond the Sword expansion packs. With Protective leaders the Archery and Gunpowder Units receive Drill I and City Garrison I automatically. Protective leaders also get double production speeds for the Walls and Castle.
The following leaders are Protective:
- Tokugawa (Japan)
- Gilgamesh (Sumeria)
- Mao Zedong (China)
- Wang Kon (Korea)
- Qin Shi Huang (China)
- Sitting Bull (Native America)
- Saladin (Arabia)
- Charlemagne (Holy Roman Empire)
- Churchill (England)
The Protective trait is generally more effective in the early game, where cheaper Castles and Walls are helpful to build a fast defense in the event of war, and highly promoted Archers and Longbowmen help prevent early attacks.
In the mid-game, gunpowder units are used both in offense and defense. Drill I can be quite useful for offense (as it can lead directly into promotions such as Pinch), while City Garrison I means no matter which gunpowder units take a city, they can keep it defended.
In the late-game, however, the Protective trait becomes weaker as combat diversifies, your quick-building buildings are obsolete and the trait doesn't help you with late-game victories.
So, to make the best use of the Protective trait, it may be a good idea to keep defensive early on and launch an attack in the Renaissance, or in the Medieval era if the civilization you're running has a capable unique unit for the time.