The Border (formerly known as the "California Border Quarantine Facility") is a human facility located somewhere in the Sierra Nevada range that serves as the headquarters for the paramilitary faction known as "Alpha-Omega". It served as the main location of the second half of the third film, and it was destroyed by Caesar during the final battle released during it's climax.
The complex is located in the entrance of a cave at the foothill of a mountain range surrounded by a large plain on it's front.
It was formerly used to quarantine people who suffered from the Simian Flu Pandemic. Before the pandemic, according to Colonel McCullough, it was a weapons depot. When it was converted to a quarantine facility, the weapons were not removed and instead stored in the mountain.
Caesar discovers the area when he finds a dying Spear and several other apes tied to crosses, he approaches Spear to ask what happened. Spear tells him the rest of the ape clan has been captured and are being forced to build a wall with no food or water, Caesar tries to ask about his son Cornelius but Spear succumbed to his wounds and died. As Caesar looked to see the camp, he is caught off-guard and captured by Red.
Caesar was taken into the camp where he meets the Colonel who has Caesar escorted through the camp, where he see the apes captured and spots his son Cornelius calling to him but doesn't respond to protect him from danger. Caesar is placed in a cage where he wallows on his failure of his colony who seem to have lost faith in him.
The apes were forced to work on a wall to create a barricade. Though imprisoned, Caesar still inspired his people by speaking against the humans' torture when an orangutan was being punished. Caesar eventually learned that Alpha-Omega was considered a rogue faction for Colonel McCullough's killing of his own men. As a result, the Northern Military was expected to attack with Alpha-Omega planning to defend themselves using the weapons left over from when the Border was a depot.
As the apes made their escape from the facility, the expected attack came. After becoming infected with the mutated Simian Flu, Colonel McCullough committed suicide. With Alpha-Omega targeting his people, Caesar detonated a large fuel tank, destroying the defensive wall. The explosion created a chain reaction that consumed the base and most of Alpha-Omega. The remains were taken by the Northern Military shortly thereafter, but the explosion triggered an avalanche. The avalanche consumed the base and the human survivors of the battle.