Bunker

Station human troops inside your bunker to help defend your base. These troops remain in the cover of the bunker and fire at attackers from this fixed position.

Details
The bunker is a defensive building unlocked first on Command Center 5. and you can get one more at Command Center 8. The units which can be placed inside have to be human troops, who will shoot from the inside so it is recommended to choose troops with high DPS to put inside such as Snipers or RPG Troopers and RPG also have aoe damage by making them the best troops in bunkers.