Gates are important, protecting entrances to regions, cities, strongholds, portals, treasure, etc. and thus must have a guardian to protect them. These guardians can be solitary or numerous, human or animal, normal or mystical, but will always serve as a tough impediment to any who try to cross the gate uninvited.
Supertrope to Angry Guard Dog, and not to be confused with Threshold Guardians, although a gate guardian can also be one of those. A gate guardian often invokes You Shall Not Pass by nature of his job. Related to NPC Roadblock, compare and contrast Locked Door. A Beef Gate can be a literal gate guardian, but not always.
In the Bleach manga, the Seireitei, the center of power within the Soul Society has four gatekeepers, one for each of its gates: Jidanbo (West), Higonyudo (South), Danzomaru (North), and Kaiwan (East). Jidanbo is a giant who is not a fantastic fighter, but his massive girth is necessary to open the gate, which only responds to physical force and resists spirit energy.
Oimo and Kashii from One Piece initially start off as this during the Enies Lobby Arc against the protagonists before making a Heel-Face Turn.
The Night's Watch guard the passages through the Wall, a 700 feet high and 300 miles long structure that separates the North from the the lands beyond the Wall. They fight off repeated assaults from Wildlings attempting to invade Westeros, but the true threat they guard against are the Others.
The Bloody Gate, the entrance into the Vale of Arryn, is guarded by a small garrison under the command of the Knight of the Bloody Gate, who traditionally asks "Who would pass the Bloody Gate?" to any who seeks passage through. At the start of the series, the position is held by Ser Brynden Tully, but he resigns the post to fight for his grand-nephew in the War of the Five Kings. He is replaced by Ser Donnel Waynwood, who is directed by Lady Lysa Arryn to prevent any outsiders from accessing the Vale while the war is going on.
Also in The Vale are the Gates of the Moon, which protects access to the Eyrie, the seat of House Arryn. The defender of the Gates of the Moon is given the title Keeper of the Gates of the Moon. Lord Nestor Royce holds the title at the start of the series, and in A Feast for Crows, the title and position are made hereditary to his line.
Access to the region of Dorne by land is restricted to two mountain passes: the Boneway and the Prince's Pass. The Boneway is defended by a castle at Yronwood, seat of House Yronwood, with the head of the house given the title Warden of the Stone Way (the colloquial name for the Boneway) and is tasked with defending the gate at the end of the pass. Their sigil (a black portcullis) and house words ("We Guard the Way") reflect this duty. The Prince's Pass is similarly defended by House Fowler from the castle of Skyreach, and they are given the title Warden of the Prince's Pass.
Moving Pictures, a pastiche of film history, has the golden man, an obvious Shout-Out to the Oscar, who is at first is taken by the characters to be some kind of dangerous god sealed away behind a gate. Then they realise that the text with this information should be read right to left, and he is in fact standing in front of the gate, i.e. as a guard against the Eldritch Abominations on the other side.
In the Angel episode "A Hole in the World" we meet Drogyn, the keeper of the Deeper Well, a prison for ancient dead demons. He guards the entrance to the Deeper Well, hidden in a tree in the Cotswolds.
Cerberus is the guardian of Hades, preventing the dead from leaving as well as the living from entering the place.
Tartarus, the deep abyss of Hades, used to be guarded by a female dragon, Campe, before Zeus killed it to free the giants imprisoned there. Later the hundred-armed giants, Hecatonchires, became the new guardians. In Roman mythology, however, Tartarus was actually guarded by a hydra. Tisiphone of the Erinyes (also known as the Furies) was also said to keep guard on the top of a turret, slashing the prisoners with her whip.
In common Christian depictions Saint Michael is the guardian of the gate of Heaven. He often looks in a book to see whether a soul has been good and can enter, or bad and has to go to Hell.
In Norse Mythology, Garm was a bloodstained watchdog that guarded the gates of Hel (the Norse land of the dead).
In the Yu-Gi-Oh! Trading Card Game, there's "Gatekeeper", a mechanical creature who seems to guard dimensional gateways. There's also "Gate Guardian", the Paradox Brothers' ace monster in the anime. Presumably it guards gates with its mighty power.
In Devil May Cry 3, Cerberus is the guardian of the entrance of the Temen-ni-gru who prevents humans from getting in. Agni & Rudra guard a specific door inside temen-ni-gru to prevent further progress of anyone who got past Cerberus.
In Dynasty Warriors, Dong Zhuo's officers Hua Xiong and Lu Bu are in command of guarding Sishui and Hulao Gates, respectively, when the allied forces attempt to remove Dong Zhuo from power. Defeating, or avoiding in the case of Lu Bu, these two officers is usually an objective in one of the early stages of each games campaign.
Hong Meiling from Touhou guards the gate of the Scarlet Devil Mansion. Fandom has flanderized her as spectacularly inept at her job, often sleeping all day long and punished for it by Sakuya.
The Gatekeeper in the The Elder Scrolls IV: Oblivion expansion The Shivering Isles is a giant monster created by Relmyna Verenim out of the flesh of the dead to guard the exit from the Fringe out into the rest of the Shivering Isles. The player kills it to gain access to the rest of the realm, but is later tasked by Sheogorath to create a new Gatekeeper with Relmyna's help.
One episode of Samurai Jack has Jack discovering a working time portal, and a guardian who is sworn to not allow ANYONE to use it unless they are worthy. Jack's repeated pleas that he needs to use the time portal to defeat Aku is met with general apathy from the guardian. Jack finally decides to take the portal by force, and battles it out with the guardian. In the end, Jack is defeated and is politely told to leave, which he does. The guardian looks into the portal and sees an older Jack leading an army against Aku. He says that Jack wasn't worthy of using the portal. Not yet...
Five hats means that five tropers think it is ready to publish.
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it doesn't duplicate an existing trope, there are at least three examples, and the title makes sense.
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