What should you do when the infamous "barbarians" flee from a greater evil? Build a wall or barrier wide enough to keep them at bay. You know, it should be strong enough to withstand the bigger baddies as well. What Could Possibly Go Wrong?
Well, it is still possible to exploit a natural breach to weaken and bring it down with Siege Engines. Perhaps because the enemy utilized a Fantastic Nuke, or perhaps through a regular old Battering Ram. After all, a defense is only as strong as its weakest point.
Sometimes, it's quite common to imagine the baddies would have solved much of their problems by bypassing the actual obstacle. Anyway, the countless hordes of the incoming threat can trigger the story plot by this mere action. May lead to a Seal the Breach plot in response.
Compare Barrier-Busting Blow, which is at a much smaller scale; There Was a Door, when a single person bursts through a less fortified, often interior barrier; and "Open!" Says Me, when a burly character uses force to break through a locked door.
- Happens twice early on in Attack on Titan, where the Colossal Titan and Armored Titan breach Wall Maria, and then, just a few years later, the Colossal Titan breaches the gates of the Trost District, letting the lesser Titans into humanity's last refuge. Plugging the hole in the Trost walls is the main goal of the following arc, while finding a way to close the Wall Maria breach and to retake the land from the Titans is pretty much the Myth Arc of the manga.
- Viking invaders from Prince Valiant lay siege to Camelot, and use a known and proven method of breaching heavy walls: they raise an armored roof over a crew of tunnelers, who dig beneath the wall. Once enough earth has been cleared away, the support beams are set alight, causing them to buckle. This in turn causes the wall to collapse from its own weight, creating a sizable rift that the Vikings clamber through to capture Camelot.
- In The Lord of the Rings: The Two Towers, the first part of the battle at Helm's Deep has Rohan's army easily keeping Saruman's Uruk-hais at bay. Unfortunately, no one's aware Saruman has found out the fortress wall can be destroyed by placing a bomb on a sewage drain nearby. Guess what happens a few minutes later...
- In Pacific Rim, the Humongous Mecha were set to be decommissioned after a huge wall was built around the pacific ocean to keep the giant alien monsters out. It took a kaiju about an hour of sustained bashing to get through, demonstrating how useless it is as a long-term solution.
- Avengers: Infinity War: During the Battle of Wakanda, the Outriders try to Zerg Rush the Wakanda barrier. Due to their sheer numbers and speed, at least a few of them manage to tear holes in the barrier and pass quick enough before it regenerates and turns some unlucky troops into Ludicrous Gibs. After the Outriders start to fan out along the barrier, which would allow them to breach multiple fronts, T'challa orders to create a slight opening in the barrier so that the enemy forces go through there, creating a bottleneck situation. From there, he, his army and his fellow superheroes start to charge at the incoming Outriders.
- Harry Potter and the Deathly Hallows: When Voldemort's Death Eaters attack Hogwarts, the professors conjure up an impenetrable protective bubble around the castle that incinerates anyone who tries to breach it. It only gives the Hogwarts crowd a brief reprieve before Voldemort shows up in person with the Elder Wand and singlehandedly destroys the shield spell. However, it's obvious that Voldemort overtaxed the Elder Wand in the process, which begins to show visible cracks.
- The Ko-Dan Armada from The Last Starfighter has been kept beyond the frontier of the civilized galaxy by an energy wall generated by thousands of satellites in a grid formation. That is, until the Ko-Dan laser-bore a hole in this wall, through which they launch meteors at Star Base. The damage created by these meteors disables the satellites, so the frontier wall dissipates, allowing the armada free go at the galaxy.
- In David Gemmell's Legend, the fortress of Dros Delnoch protects a vital mountain pass that is the only practical way for an army to invade the lands of the Drenai from the Nadir Steppes. The fortress is composed of seven walls that span the pass with each wall representing a stage of The Siege. The first two walls are the least defensible and are assumed to fall if assaulted by an army of sufficient size and competence. Walls three and four are the ones where the most brutal fighting takes place as both the attackers and defenders have been bloodied and hardened by the fighting on the earlier walls. If the first four walls are breached then it is likely that the fortress will fall without outside intervention. Walls five and six represent the bravest and most determined defenders performing Heroic Sacrifices so their comrades can retreat. Wall seven is where the Last Stand will take place.
- The Wildlings from A Song of Ice and Fire repeatedly attack the eponymous Wall in order to travel south. Much to their grief, the Night's Watch makes their efforts a living hell, to say nothing of the fact that it's a 700-foot-tall expanse of enchanted ice. A few of them (including Osha) are clever enough to bypass it and reach Winterfell, though. It is implied in the books the White Walkers have the power to destroy it and march on Westeros.
- Paul's attack at the end of Dune has him nuking a hole through the Shield Wall: a whole mountain range protecting (and enabling the existence of) the most hospitable area of Arrakis.
- In the Sword of Truth series, there were two cases of magical barriers put up in order to stop a war. The first is broken by the Big Bad of the first book in order to have an easier time conquering and looking for the McGuffins, the second is deactivated by the protagonist so that he can stop The Keeper from breaching the barrier between life and death, destroying everyone.
- In the The Belgariad sequel novel Guardians of the West:
- When The Hero Belgarion leads an assault on a fortified city that's been taken over by Bear-Cult rebels, he uses sorcery to break down the city gates. He forgets that he's holding the Orb of Aldur, a tremendously powerful Amplifier Artifact that's quite enthusiastic about helping him, so the spell instead vaporizes the gates and sends chunks of gate tower flying miles away.
- Having learned a bit of subtlety, the next time Belgarion goes up against a Bear-Cult stronghold, he uses his sorcery to breach an aquifer under the city and raise the water table. After a night, the walls anticlimactically fall over in a minor mudslide and his forces march in.
- As an officer in Wellington's army during The Napoleonic Wars (and slightly beforehand, in India), Richard Sharpe participates in several major sieges and personally charges through a gap in the walls created by artillery or sappers on at least two occasions.
- Game of Thrones: The dreaded villains of the series, the White Walkers and their army of wights, have been barred from invading Westeros for thousands of years due to a massive wall of ice and stone that has some form of magic that prevents their legions of undead from breaking through. That was until the final episode of season seven, when the leader of the White Walkers named the Night King uses a reanimated dragon to blast fire through the wall. Almost immediately the easternmost section of the ice wall began crashing down, thus allowing the Night King and his comrades and soldiers to cross the millennia-old structure and finally unleash their wrath on the vulnerable Seven Kingdoms.
- Rise of Empires: Ottoman:
- Constantinople's harbor is defended by a massive cast-iron chain that reaches all the way to the colony of Galata. The Ottoman forces manage to break into the Golden Horn by moving their fleet through the mountains north of Galata.
- Mehmet's army finally breaks through Constantinople's walls after a Rousing Speech and what appears to be a sign from God, with the Janissaries eventually making the final push that overwhelms Giustiniani's forces.
- According to the Book of Joshua (6:14 and 6:15), the Israelites sundered the ancient walls of Jericho, by chanting at it and blowing rams' horns. Archaeologists have dated the foundations of Jericho's walls to the middle Bronze Age, and discern where it underwent many repairs, rebuilds and upgrades in its history.
- The Achaean (Greek) armies had laid The Siege to the city of Ileum in Virgil's The Aeneid for several years, plundering the surrounding province of Troy, but not getting past the walls of the city proper. That is, not until the Trojan Horse ploy that installed thirty or so Elite Mooks behind Ileum's walls, where they unbarred the gates, allowing the Achaean army access.
- The results are seen after the fact in The Elder Scrolls IV: Oblivion. Mehrunes Dagon's forces attacked Kvatch with a Daedric Siege Crawler that blasted open the city's walls. They attempt it again at Bruma later in the story, but the Player Character closes the Oblivion gate on the Crawler, destroying it before it can reach Bruma. The whole game is actually predicated on this trope, as the assassination of The Emperor Uriel Septim VII and his heirs allows the Daedric armies to breach the mystical barrier between their native Oblivion and the material plane of Nirn and to stage an invasion through hundreds of Oblivion Gates that opened across Tamriel as a result. The main quest of the game revolves around finding a way to mend the veil between Nirn and Oblivion and thus to prevent any more Gates from opening.
- Dragon Age: Inquisition opens with the Hidden Villain breaching a massive hole in the Veil that separates the Spirit World from the material plane. With demons now pouring from the Breach in droves, finding a way to close it is the first major objective of the game.
- The Soviet campaign of World in Conflict: Soviet Assault starts with you demolishing a part of the Berlin Wall with precision artillery strikes, so Soviet tanks can squeeze through and mount an attack. This is mostly a symbolic action, however, since the Soviets also mount simultaneous attacks elsewhere along the Iron Curtain.
- Horizon Zero Dawn: Aloy helps her War Chief track the people who attacked her tribe back to their base. She sneaks in and blows up thier stockpile of explosives taking down a wall and allowing the Nora to bypass the fortified main entrance.
- During the assault on the Last Judgement's compound in Rebuild, the attackers blast a hole in their walls letting them contend with zombies in addition to the humans.
- A Scotsman in Egypt: Happens repeatedly over the course of the many, many sieges the Scots lay to their enemies: their catapults/cannon concentrate fire on the gate or a wall section, and the Scottish hordes pour into the enemy. That is, when their spies haven't infiltrated the city to open the gates for them.
- Avatar: The Last Airbender: After years of sieges against the Earth Kingdom capital of Ba Sing Sae fail to breach its massive outer wall, the Fire Nation builds a giant armored drill to bore through it. Fortunately for the Earth Kingdom, the Gaang arrives at the city right at that time and are able to disable it, but not before it goes all the way through.
- It happens again in the series finale, where Iroh, his firebending amped up by the comet, is able to breach the wall in a single attack.