Land of One City

And you said a country can't have one city...

When a country is a city-state, i.e. the city is the country, or the other way around: the country has only one city.

Although this can be justified by the country being small (e.g. Monaco, Vatican City, Singapore), it is not justified when a country of this size has an economy the size of the US economy. This is a perfect opportunity to mix this trope with a hearty helping of City of Adventure. In Anime a city like this is usually a Utopia or Dystopia.

Note that the area with one city does not have to be a country, it can be a planet or county or etc...

Examples of Land of One City include:

Anime and Manga

In Appleseed, Olympus, the most powerful country, apparently has only one city.
Almost all (or maybe all) the "countries" in Kino no Tabi are like this.
- Even if there are a few that aren't, Kino never stays long enough to find out.
At the end of Super Dimension Fortress Macross (or the first season of Robotech), humans have been nearly wiped out by the Zentradi, and have only one city left. Nonetheless, they still field a very powerful military.

Comic Books

The various Mega-Cities in Judge Dredd each only have direct authority over their respective urban areas, though MC-1 at least does have some influence over parts of the Cursed Earth.
Attilan, home of The Inhumans.

Film

The Matrix had Zion, the last human city.
Star Wars: Coruscant is a city the size of an entire planet.
- Its presumed to have had multiple cities at some point in the distant past. Now, like with New York City and London, several once-separate municipalities have been devoured by the more dominant one and turned into boroughs, albeit on a slightly larger scale than either of those.
In the live action Super Mario Bros. movie, the parallel world where Koopas reside had only one city, surrounded by endless tracts of desert.

Literature

A few locations in Discworld are city-states, most notably Ankh-Morpork and Genua.
Trantor from the Foundation universe is a planet whose entire surface has been urbanized, thus making it one big city.
Tar Valon from The Wheel of Time; also, the (less-important) city-states of Mayene and Far Madding.
Grantsville in 1632 is effectively this. Interestingly there were a number of independent cities like that at the time. Unfortunately they didn't possess repeating rifles.
Perdido Street Station and Iron Council feature New Crobuzon, which in the latter book is at war with the city-state of Tesh. The Scar also features a city-state made of pirate ships stuck together.
The crumbling city of Mervyn Peake's Gormenghast appears to be the only city in the world. In fact, it seems to be the world.
The Queens in Septimus Heap rule only over the Castle.

Tabletop Games

Magic: The Gathering has Ravnica, a plane with only one city. That said, that one city is large enough to occupy the entire plane.
- The only city we see in the set Amonkhet is Naktamun. There is a very real possibility that Nicol Bolas destroyed, directly or indirectly, all of civilization on Amonkhet with the exception of Naktamun.
Lookshy, a relatively small city state, is capable of fielding military forces comparable to the Realm (which on its own is an island the size of the continental United States and recieves tribute from across the world), through a combination of an extremely militant society and huge stockpiles of artifact weaponry. Lookshy is comparably disadvantaged in that it doesn't have nearly the same power projection as the Realm (they can protect themselves and their neighbours, but are unable to be as expansive).
- Nexus has economic power comparable to the Realm, partially because it is at the heart of the Scavenger Lands (and benefits from some protection by Lookshy) and partially because it serves as the headquarters of a powerful, worldwide mercantile guild. The Emmissary is also important in maintaining the autonomy and power of the city.
The Dark Sun setting for Dungeons and Dragons has several city-states, each of which controls one of the few remaining fair-sized spots of fertile land and not much else (centuries of sorcerous warfare and use of magical WMDs millenia past reduced most of the world to desert, natch).
In Planescape, Sigil is technically part of the Outlands, but is physically separate from the rest of it, being something of a small Plane of its own. It's clearly independent and self-governing. The Gate-Towns of the Outlands (which vary in size from hamlets to small cities) are also self-governing city-states.
Infamously bad roleplaying game The Spawn of Fashan includes a Fantasy World Map covering an area about the size of Australia, in all of which there are only four cities.

Video Games

As revealed in Donkey Kong Land, Donkey Kong Island has one city: Big Ape City.
Final Fantasy IX has Lindblum and Burmecia, which control a goodly portion of the continent with only one city. Also, narrowly averted in that Alexandria has a whopping three cities (or two, it's ambiguous where Treno stands)
Final Fantasy XI's nations of San d'Oria, Bastok, Windurst, and Jeuno are all technically city-states in the present time, although the first three have historical areas that they controlled in the past that are now up for grabs. Aht Urhgan is seemingly the only in-game nation with actual vast stretches of territory.
The now-destroyed Zendar from Mount & Blade.
Tropico 3 has only one city to build on, and you CAN STILL have a thriving economy.
World of Warcraft: Although it's not very clear in the game itself at first glance, according to lore Stormwind is actually a city-state, and the adjacent regions are independent governments that just allow Stormwind to protect them with its military. Of course, the same lore also claims that Stormwind has 200,000 citizens (more than every other city in the game combined, despite being physically smaller than several of them), so this might be taken with a grain of salt.
- Pretty common in video games with world maps due to Conservation of Detail.
- The 200,000 people issue has been more or less confirmed as an error, reducing it to the more realistic 20,000.
Rapture isn't especially large compared to most real-world cities, but it's definitely self-contained, self-sufficient, and a sovereign nation as far as anyone's concerned. Things like beef and tobacco do have to be smuggled from the surface (though it's never explained why), but they can get by on artificial substitutes when they have to.
4X games typically start with every empire consisting of only single city. Though establishing or conquering additional cities is usually the first thing the players do.
What are commonly called One Province Minors(OPMs) in Europa Universalis are these, states with only one province, and thus, in game terms, only one city.

Web Original

The eponymous village of The Questport Chronicles
Metamor Keep consists of the titular keep and a few outlying villages. By the time of Metamor City the keep has developed into an arcology covering most of its original territory and is the capital of an empire spanning most of the continent.
The Kingdoms of Remnant all appeared to be this in RWBY, at least until the World of Remnant supplementary background videos revealed that the "protected" human enclaves in this Death World were actually fair-sized territories with multiple (as-yet unnamed) cities in them.

Real Life

Monaco and Singapore are Real Life examples, while the Vatican takes it Up to Eleven by being an enclave within the city of Rome. That makes this trope Truth in Television.
Similarly, Hong Kong and Macau are not exactly states, but they're self-governed enough to be considered as such.
With regard to other countries:
- Kuwait doesn't have very much besides its eponymous capital.
- France is an interesting case: while it's a big country with plenty of populated places, its only city with a population of more than a million is Paris (2.2 million in the city proper, and 12 million in the entire metropolitan area). The only other city within striking distance is Marseilles (900,000), and all other cities are smaller than 500,000. This has the effect of making France seem like it's a Land of One City, even though it's far from it.
- More than one third of the Greek population lives in the region of Athens.
- Almost 40% of the Republic of Ireland lives in the Greater Dublin Area.
- Iceland has only one city, Reykjavík. Approximately one third of the country's population lives in Reykjavík, and another third lives in the suburbs of Reykjavík (Álftanes, Garðabær, Hafnarfjörður, Kjalarnes, Kópavogur, Mosfellsbær and Seltjarnarnes). That leaves the last third with the entire rest of the country.
- Half of the population of Uruguay live in Montevideo.
Truth in Television historically. Ancient Greek city-states were just that, cities that controlled relatively small amounts of the surrounding land. Similar conditions prevailed during large parts of Medieval Italy, co-existing with larger realms like the Papal States or the kingdom of Sicily.
- The Italian city-states, despite their name, actually subvert this. Most were the size of small countries, incorporating numerous surrounding settlements and townships. They were only city-states to the extent that power was concentrated in the hands of the eponymous cities, such as Florence and Venice.
- Actually, ancient Greek poléis had other towns in their territory. They were called démoi. The important thing in a Greek city-state was not the city, but its citizens. A citizen of Athens could live outside Athens, in a small village at the other side of the Attic region, but he was still Athenian.
- Also many of the smaller states that made up the Holy Roman Empire.
- Like many steepelanders the Kievan Rus had these. Notable among them are Kiev and Novgorad. City-States often arose in Central Eurasia because the local soil is usually unfit for cultivation and therefore the chief way to survive, besides nomadism, was to gain (or create) dominance of traffic points that allowed a settlement to live by imports and only cultivate a limited amount.
New Zealand has a population of 4.3 million, with more than a quarter of those people living in Auckland (1.3 million). The next largest cities are not even a third its size.
North Korea, Pyongyang is the only city of any significance. Satellite photos shows that its only city that has any lights the rest of North Korea are pitch black [1].
Common in the Middle East and Central Asia. While they might or might not pay nominal allegiance to a far away emperor, the desert or steppe made any oasis like an island in the ocean and a city on that oasis would likely rule it practically independently.