Code Geass: In season two the Black Knights take refuge in the Chinese Embassy in Tokyo, and even claim one of the embassy's rooms as their sovereign territory. The embassy's smack dab in the middle of territory controlled by the vastly, vastly superior Brittanian military; the only reason Brittania doesn't just attack the Black Knights is because invading a foreign embassy is enough of a diplomatic faux pas that it'd probably just start another war. Bit of an unusual example, though, in that the Black Knights can apparently escape unnoticed any time they wish, but prefer to keep the Brittanians distracted by the embassy for as long as possible.
One takes place during episodes 16-18 of Tears To Tiara, when Gaius of the Divine Empire tries to take Avalon, where the demon king Arawn is at.
The ongoing story of Attack on Titan has the Survey Corps trying to hold off the Titans as they assault the last city of humanity.
Of course, Siege is about a siege. Asgard, the city of the gods, is under siege by the Dark Avengers, H.A.M.M.E.R. and an army of superheroes of the 50 states initiative. Then The Cavalry (the good Avengers, and Patriot) shows up, but Asgard was destroyed by Sentry.
Issue 175 of Sonic the Hedgehog had Eggman not only pull off a siege on Knothole, he razed it to the ground.
Saving Private Ryan: Uses this trope in the climax. When Miller's squad find Ryan, he refuses to abandon his mission to hold a strategically vital river crossing for the Allied invasion, in spite of the airborne troops on the location lacking leadership and being outnumbered and outgunned by advancing German forces. Miller decides to lead the defense and let his men join the action not because the bridge is their responsibility, but so that they can ultimately bring Ryan home.
John Woo's The Killer features one of these as its final shootout, with the title character and his Cowboy Cop ally defending the Killer's last place of sanctuary, a church, against a virtual army of assassins sent by his ex-boss to murder them all.
Scarface ends with a siege by assassins working for Alejandro Sosa against Tony Montana's mansion, which doesn't really get going until Tony takes up an M-16 with a grenade launcher with a cry of "Say hello to my little friend!"
The Siege, usually involving a Town Boss being held in a city jail, was the climactic event of four John Wayne movies, including Rio Bravo, El Dorado, The Sons of Katie Elder, and Rio Lobo. Apparently Duke liked this story line even more than he liked stalking and spanking beautiful women (three different movies!).
Defied in Pirates of the Caribbean: At World's End. Several of the pirate lords advocate hiding in their fortress when the East India Company comes calling; said fort is supplied for a several-month siege. Jack points out that they could do that, but half the fort's inhabitants would be dead in a month due to cabin fever-induced civil war.
Evil Dead. In the 1st and 2nd movie the demons attack the wood cabin where Ash and his allies reside. In the 3rd movie the skeleton army assault the castle.
Skyfall climaxes with Bond and two others (Kinkade, M) defending a manor home against more numerous and better-armed villains. Defenders' advantage allows the MI-6 contingent to hold out against 10-to-1 odds, but they technically still lose because the Big Bad is able to achieve his mission objective (killing M)—or, at best, stalemate, since Bond kills him right after. (Also, the home gets blown up.)
The Silmarillion features the Siege of Gondolin, except unlike the above examples, Morgoth's forces succeed in taking the city, with only a remnant of its population escaping.
Also shows up in The Hobbit when Thorin and his band are holed up inside the Lonely Mountain as the armies of Laketown and the Wood Elves try to get in to claim the treasure.
Dan Abnett's entire Warhammer 40,000: Gaunt's Ghosts novel Necropolis is one prolonged siege battle, with the Chaos-corrupted hive city Ferrozoica hurling their entire population at the much larger but much less-militarized hive city Vervunhive. Think Minas Tirith, but with tanks and a good hundred times the manpower. It occurs again in Sabbat Martyr, with Gaunt explicitly comparing and contrasting the two situations, noting that this second time around the "good guys" were even worse off.
Also in the game background the Ultramarines' defence of their polar fortress againt the Tyranids that had invaded their home planet. Most of their 1st company died holding the place untill the Imperial Navy could drive off the Hive Fleet by having a battleship perform a Heroic Sacrifice and explode it's warp core right in the middle of the fleet.
Noticeably averted in War and Peace. Kutuzov abandons Moscow despite everyone on his staff and his emperor demanding that he hold Moscow against a siege.
A mainstay of Henryk Sienkiewicz's ''Trilogy'', set in the 17th century Polish-Lithuanian Commonwealth. Each of the books in the trilogy has a climactic siege featuring the defenders fighting against great odds.
Rogue Male: for a large part of the story, the hero is besieged alone in his hideout, which has gone from a refuge to a hellish trap.
The climax of the entire Harry Potter series takes place in the last third or so of Deathly Hallows, when Voldemort and his Death Eaters storm Hogwarts itself.
In the book Azincourt by Bernard Cornwell it goes into great detail over the siege of Harfleur. The siege is a shambles, the British end up with dysentery, the French keep rebuilding the walls and to top it all a ship gets past the blockade to resupply them. When King Henry V finally takes the small town he lost so many forces he can't possibly hope to defeat the French in open battle but to save face forces his men to march around France attempting to avoid their army...that doesn't work out so well.
Colas Breugnon has a siege described early in the novel. Even though a few people die, both sides come to an agreement after a few days and settle down to eat together. No one's very sore about the entire incident.
The original Mistborn trilogy has a few such examples.
In Mistborn: The Well of Ascension the protagonists are besieged in Luthadel by three armies at once; the fact that the besieging armies are all working against each other is the only reason Luthadel lasts so long.
In Mistborn: The Hero of Ages, the good guys are the ones doing the besieging, until the Big Bad's army shows up and the besiegers and the besieged decide to team up.
Occurs several times in the Malazan Book of the Fallen, most notably the exceptionally one-sided siege of Capustan in Memories of Ice.
Defied in the first book of The Deed of Paksenarrion. Paks's unit is holding a fortress when another mercenary company shows up with siege engines. Paks's unit is not equipped or supplied for a siege (and the other side is a sometimes-allied business competitor, not some Army of Evil), so they surrender.
The main conflict in Redwall is a siege laid upon Redwall Abbey by a vicious one-eyed rat named Cluney the Scourge. Cluney's forces actually manage to get in and have to be thrashed afterwards.
There are many, many other sieges in the series, either against Redwall or the Badger Lord's fortress of Salamandastron.
Part of the plot of Septimus Heap: Darke involves the Siege of the Wizard Tower by the Darke Domaine.
There are many sieges in A Song of Ice and Fire. One siege at Riverrun is entirely at a stalemate until Jaime comes and dissolves the whole mess. Daenerys becomes fairly well acquainted with sieges throughout Slaver's Bay, having been on both sides of them. One of the most memorable sieges in the series is in the back story, the famous Siege of Storm's End in which Stannis's men were nearly dying of starvation toward the end. This gave Davos his opportunity to become a main character by smuggling onions into the castle.
In the backstory to Warrior Cats, SkyClan had to endure this. Forced out of their home, they found a gorge and settled down in it. However, a massive horde of rats surrounded them, just waiting for them to try to leave the get some food, take a nap outside the gorge, or something of the sort. When a cat left, they were swarmed by rats and killed. It was enough to drive a cat mad, and led to the end of SkyClan. However, the cats get the last laugh, as the rebuilt SkyClan drives a group of rats under a pile of garbage, surrounds it and waits for them to try to escape, then kills them as they leave.
Late in Freedom anti-Daemon mercenaries carry out one against a Daemon community.
The first series of the revival ended with a Siege, though it was slightly subverted in that the good guys did not actually end up holding off the evil Daleks, and by the end of the episode, every main and minor character, with the exception of Rose Tyler, was dead. Two getbetter.
"The Base Under Siege" is a standard Doctor Who plot ("Moon base, sea base, space base, they build these things out of kits!"), especially in Pat Troughton stories, such as "The Wheel in Space"
"The Waters of Mars" subverts a number of conventions in this regard.
One specialty of the Imperial Fists Space Marines in Warhammer 40,000 is holding the line in sieges. Not coincidentally, the specialty of their arch-enemies, the Iron Warriors, is launching them.
In the Horus Hersey the Chaos Space Marines lay siege to Terra, and the Emperor along with his remaining forces were are forced to drive them off with all they have. The Imperial forces have been driven off the Chaos forces, and killed Horus, but at a terrible price, Sanguinius has been slain, and the Emperor is in a near death state, and is locked in life support in the golden throne.
Several attempts were made in the golden age of table top wargaming to make siege games. The problem was that there is little maneuver in siege warfare and the most interesting parts are the gadgeteering which doesn't translate well. Some did manage to become classics like Avalon Hill's Siege of Alessia
In Checkers during an end game, if the vanquished player is stubborn about it, he can retreat to a double corner. It needs a carefully choreographed sequence of moves to dig him out and is rather like a siege.
The old Norse game Hnefatafl simulates an escape from a besieged fortress, as opposed to a pitched battle(like Chess).
Happens in Neverwinter Nights 2 when the hero's castle is besieged by the undead army of the King of Shadows. The already problematic odds take a turn for the worse when one of the hero's companions turns out to have betrayed them by sabotaging the gate and when the undead, including the vampires, turn out to be unaffected by sunlight.
Mass Effect 2: Garrus' recruitment mission and Grunt's loyalty mission both consist of three or four heroes holding a somewhat defensible position against a horde of mercenaries/alien monsters, followed by a Background Boss. Legion's loyalty mission is a sort-of Tower Defense scenario.
Urban Dead: The game revolves around humans building barricades inside buildings and zombies trying to break in. Most famous is probably Second Siege of Caiger Mall, going for three real life months.
Goblin sieges in Dwarf Fortress; more rarely, human and elven sieges. If all the resources you rely on are subterranean (water, magma, farmlands, wood, ores,) a virulent forgotten beast can effect a siege from below.
In the PS3 and 360 versions of the first The Godfather game, a slew of Cuneos assault the Corleone compound, and your job is to hold the fort with your fellow mobsters. These become more common in the sequel since enemies can now randomly raid your fronts.
Happens to your castle at the end of Dragon Age: Origins – Awakening. Depending on your administrative decisions during the campaign, if you stay to protect the city instead of returning to defend the castle, it can be either a Last Stand for the defenders, or a successful repulse of the siege.
Crossed with Delaying Action in the third Terran mission of Starcraft: you have to hold your fort against repeated Zerg attacks until dropships arrive to evacuate your troops.
Act III of Diablo III has you defending Bastion Keep in Arreat against a full-on demonic onslaught by the forces of Azmodan.
Happens a lot in pretty much every installment of Fire Emblem, defending for a certain number of turns in some levels and seizing castles in the seige missions.
During the Cataclym expansion of World of Warcraft, the Dragon Soul raid is this. All of the Twilight's Hammer and the Black Dragonflight are bringing their forces to tear down the Wyrmrest Temple and bring the Hour of Twilight. It's up to you and 9/24 other heroes to not only stop that from happening, but finally put an end to Deathwing.
A few missions in Guild Wars are based around these (Dzagonur Bastion, Thunderhead Keep, Eternal Grove, Genius Operated Living Enchanted Manifestation.)
Jak II: Haven City, such as it is, has been under siege by the Metal Heads for hundreds of years, and is believed to be the only city left on the entire planet, making it humanity's last hope. Pity that the Baron's plan to defeat them would also destroy the entire universe.
In The Order of the Stick, Azure City, a bastion of good partially ruled under the watch of paladins, is attacked by a massive army of hobgoblins led by Redcloak, an evil goblin cleric and Xykon, an evil human lich sorcerer. The protagonists find themselves defending the city alongside the paladins. The good guys actually lose with heavy casualties, including the leader of the protagonists. A very, very long arc was dedicated to the fallout of the battle, including the only recent resurrection of said leader.
On that note, Ba Sing Se itself. While the Fire Nation has made more serious efforts on occasion, the city has been under more or less constant siege for a very long time, which is why it's completely self sufficient (having a large agrarian zone inside the wall capable of supporting the entire city). This is additionally supported by the presence of Fire Navy ships in territorial waters near the city, implying that outside the city itself the Fire Nation essentially has free reign. And one has to wonder why there's no sign of plant or animal life outside the wall whatsoever, considering that the fire nation has been known to burn down villages and forests...
This trope is a case of Truth in Television. Sieges have been and (to some degree) remain a common military strategy. Modern infantry tactics favor going around strong points in the defense and let rear-echelon troops deal with them later, but sometimes there is no going around a well-placed defensive positionnote one example is from the Battle of the Bulge: the Germans' intentions were to quickly penetrate the American lines without stopping, but to be able to use the roads they had to take Bastogne (which they proved unable to do). A good number of movies and television programs base their siege plots on real life sieges like Leningrad and the Alamo. These are well-remembered by a (defending) nation's population if their people either won the siege by successfully holding their position against an overwhelming enemy or (more commonly) lost gloriously.
Back in 134 BC, the Iberian hillfort of Numantia, in today's Spain, held off a siege by the Roman Army for 13 months. In the end, the surviving defenders chose to suicide rather than be killed or captured by the Romans. To this day, the Spanish language has the adjective/noun numantino, meaning "he who tenaciously resists to the limit, often on precarious conditions."
The fortress of Masada in Israel topped that, holding out against the Romans for three years before choosing mass suicide.
During Spain's War of Independence, the city of Saragossa suffered TWO sieges by the French Army. The first (1808) ended with a Spanish victory; The second one (1809), historically noted for its brutality, ended with a French victory. Saragossa was reduced to 12,000 people from its pre-second siege population of 100,000.
The battle of Alesia was a strange example: Gaius Julius Caesar's army was besieging the city as it was the last stronghold of Vercingetorix (the last Gaulish leader against Rome), but was in turn besieges by a Gaulish relief force. The battle happened when the Gaulish relief force tried to break the Roman siege and the Alesia garrison sortied, but resulted in Caesar beating back the sortie and near-annihilating the relief force (the annihilation wasn't total only because the Romans were too tired to pursue them), causing Vercingetorix to surrender before the Romans broke in and exterminated everyone.
Lasting almost four years, The Siege of Sarajevo was the longest siege in modern warfare. It was part of the breakup of Yugoslavia, and began when, after Bosnia declared independence from Yugoslavia, Serbian forces surrounded the city in an attempt to make a new Bosnian-Serb state, which the Bosnians were none too happy about. The end result was nearly 12,000 dead, and a city forever scarred.