Camp Half-Blood has magical borders that cover the entire camp. The border acts as a shield from mortal eyes, making any mortal that looks upon it see nothing more than a strawberry field because of the Mist. The border also acts as a barrier from the weather; the Gods decide what the weather is like in camp, avoiding storms and high winds. This is mentioned by Percy multiple times throughout the books. The border will also keep any other being that isn't a demigod, unless they are invited inside the border.
The camp border's primary use however is to keep out intruders. As monsters are drawn to the smell of demigods, the magical borders were used to keep the camp safe from monsters. However, originally the borders weren't strong enough to keep out more powerful monsters, so the camp had other defenses. Hephaestus's Cabin built a large mechanical dragon in Bunker 9. The Bronze Dragon would patrol the borders of the camp and attack any monster or enemy that threatened the borders. The dragon, however, eventually went haywire and disappeared into the woods for many years.
Eventually, the demigod Thalia Grace raced to Camp Half-Blood with two other demigods named Luke Castellan and Annabeth Chase, along with a satyr by the name of Grover Underwood. While Thalia held off a large group of monsters sent by Hades, the others raced past Half-Blood Hill. Thalia however bought them as much time as she could before dying. Zeus, her father, felt sorry for her and turned her into a large pine tree at the spot where she did. The magic of the tree strengthened the overall power of the border, preventing even the most powerful monsters from entering. When the tree was poisoned, the camp borders weakened and began to fail. After the tree was healed by the Golden Fleece, the borders were restored and strengthened. As the Fleece also resurrected Thalia, it became the source of power for the borders rather than Thalia's spirit. Later, the Athena Parthenos aids in powering the borders, but its power is relative to the number of demigods under its protection.
It is unknown how the border distinguishes between allies and enemies of the camp. For example, mechanical monsters like the Colchis Bulls were affected by the shielding properties of the border. Also, several Skeleton Warriors managed to get past the magical borders. However, the borders only prevent monsters from entering the camp and monsters can be summoned inside the borders.
Camp Jupiter had several different borders that keep the camp safe, as well as the inner city of New Rome. The Little Tiber loops around the camp and is the main defense from monsters. The water will react to a monster and the current will pick up, pushing them away and, as shown with the gorgons, prevent them from reforming. The borders are upheld by Terminus, the god of boundaries. There are several statues of the god all over Camp Jupiter and they all monitor the borders. The borders of Camp Jupiter function much the same as the borders of Camp Half-Blood. The borders shield mortal eyes from the camp, as anyone that looks at the camp will only see a large open field.
Terminus himself will also see groups off when they leave the camp. Once you get into the inner city where the senate is, Terminus will prevent weapons from passing him. Even magical weapons, such as Riptide, will be kept outside the inner border and won't return to Percy's pocket.
When Percy Jackson first arrives at the borders of Camp Half-Blood, all he sees is a large strawberry field with a farm house at the bottom. His mother had told him to run past a tree to safety. However, Percy does not understand that the tree was the border to the camp or why his mother could not go past it. After defeating the Minotaur, Percy and Grover are taken inside the borders to recover. While being given a tour of the camp, Percy notices that the rain seemed to be going around the camp and did not fall within the borders. He is also told the story of Thalia's Pine Tree and how it helps to defend the camp.
Luke, after leaving the camp, manages to find a way to poison Thalia's pine tree. This results in the magical borders of the camp becoming much weaker, allowing monsters to attack on an almost regular basis. While the borders are weaker, monsters still have a hard time getting past them. During the fight with the Colchis Bulls, they slow down once hitting the border, but are still able to get past it with enough effort, unlike Tyson. Tyson, who is a Cyclops, is not strong enough to get inside to help Percy. Annabeth gives Tyson the permission to enter and the camp's borders refrain from stopping Tyson.
Because the camp's borders are so weak, the campers had set up a border patrol in order to defend against any more monster attacks, but which they know would be overrun, eventually. A quest is given to Clarisse La Rue in order to find the Golden Fleece, an object of strong nature magic that could heal the tree. Clarisse eventually makes it back with the fleece which strengthens the borders of the camp to an even higher level, while at the same time resurrecting Thalia Grace.
During the winter term at camp, the gods had allowed a small amount of snow to enter the borders. Later, several Skeleton Warriors managed to get inside the border to kill Percy Jackson, but were quickly defeated by Nico di Angelo.
Luke Castellan knows that his army of monsters could not get past Camp Half-Blood's borders, so he tries to use the Labyrinth to find his way inside the borders and attack the camp from the inside out.
When Rachel Elizabeth Dare takes Blackjack back to Camp Half-Blood to try and take on the spirit of the Oracle, Percy wonders if the magical borders would somehow force her out. However, once she arrives, the borders do not stop her and she is able to land safely.
During Percy's arrival at Camp Jupiter, the Gorgons manage to break through the camp's borders and attack Frank Zhang, until Percy used the Little Tiber to rip them apart. Later, when going to meet at a senate meeting, the god Terminus prevents him from entering. The god was able to tell that Percy had a weapon and made him leave it with Julia. Terminus keeps the pen outside the borders and Hazel explains to Percy that there are border stones all over the camp. When leaving for their quest, another statue of Terminus appears on the edge of the camp borders, where Terminus would see off anyone sent on a quest and give them directions.
Later, during the battle at Camp Jupiter, Polybotes chases Percy through the camp and tries to kill him. Percy leads the giant to the inner city where Terminus watches the borders. Once Polybotes passes the inner border with weapons, Terminus agrees to help Percy and the two kill the giant together.
When the Argo II arrives at Camp Jupiter, Terminus prevents the ship from passing because weapons are not allowed past the Pomerian Line, and the entire ship is a weapon. Annabeth Chase finds a loophole by telling Leo Valdez to have the ship hover over New Rome so that it will not technically be passing the line.
During the film, the borders of Camp Half-Blood are weakened when Luke poisons Thalia's Pine Tree. This is first indicated when Tyson is able to cross the borders without trobuble. As a result, a Colchis Bull was able to break through the barrier and wreak havoc on the camp. A quest is issued to Clarisse La Rue to find the Golden Fleece and bring it to camp to heal the tree.
|Camp Half-Blood:||Amphitheater | Archery Field | Arts & Crafts Center | Bunker 9 | Cabins | Camp Borders | Camp Store | Camp Half-Blood Forest | Climbing Wall | Cave of the Oracle | Dining Pavilion | Grove of Dodona | Half-Blood Hill | Rec Room | Sword Fighting Arena | Thalia's Pine Tree | Myrmeke's Lair | Pegasus Stables | Strawberry Fields | The Beach | Big House | Big House Attic | Zephyros Creek