1) a river island. A dangerous, difficult to cross river splits into two and the 2 parts rejoin farther down stream, making an island in the river.
A river island — on the planet Earth — can be as small as a boulder sticking out of the water or as large as the largest river islands on Earth.
https://en.wikipedia.org/wiki/River_island
The river can be surrounded by flat land and almost impassible swamps or in the mountains surrounded by high canyon walls. There could be dangerous rapids and.or waterfalls above and/or below the river island. Or maybe the river is full of boat smashing, man eating monsters.
2) A river loop. A meandering river often has a total length many times the total distance from its source to its mouth because it has a lot of loops. Many Mississippi River loops are almost islands. And sometimes a storm will cut through the narrow neck and straighten the river and make an island out of a loop. Of course the Mississippi River has been used for communication by canoe and by steamboat for thousands of years, and presumably there would be many places to land along the loop, so you will have to make the river less navigable.
3) If it can be a city, how about Venice? A group of islands in a lagoon that cannot be easily reached by water if the islands are defended. Or Ravenna, a nearby city that was surrounded by marshes and almost inaccessible when it was the capital of the western Roman Empire.
4) Perhaps a (possibly abandoned) city? This is the size of Babylon as described by Herodotus:
- Assyria possesses a vast number of great cities, whereof the most renowned and strongest at this time was Babylon, whither, after the fall of Nineveh, the seat of government had been removed. The following is a description of the place:- The city stands on a broad plain, and is an exact square, a hundred and twenty furlongs in length each way, so that the entire circuit is four hundred and eighty furlongs. While such is its size, in magnificence there is no other city that approaches to it. It is surrounded, in the first place, by a broad and deep moat, full of water, behind which rises a wall fifty royal cubits in width, and two hundred in height. (The royal cubit is longer by three fingers' breadth than the common cubit.)
http://www.shsu.edu/~his_ncp/Herobab.html
A furlong is 660 feet so 120 furlongs is 79,200 feet or 15 miles, giving Babylon a total of 225 square miles.
Of course Herodotus greatly exaggerated the size of Babylon.
Vijayanagara, capital of the Vijayanagara Empire until its terrible sack in 1565. The city center covered 40 square kilometers or about 15.441 square miles.
The whole city was full of gardens, and because of them, as an Italian visitor in 1420, Nicolo Conti writes, the circumference of the city was sixty miles. A later visitor was Paes, a Portuguese who came in 1522 after having visited the Italian cities of the Renaissance. The city of Vijayanagar, he says, is as "large as Rome and very beautiful to the sight"; it is full of charm and wonder with its innumerable lakes and waterways and fruit gardens. It is "the best-provided city in the world" and "everything abounds." The chambers of the palace were a mass of ivory, with roses and lotuses carved in ivory at the top—"it is so rich and beautiful that you would hardly find anywhere, another such.
https://en.wikipedia.org/wiki/Vijayanagara
Abdur Razzak, the Persian traveller who visited Vijayanagara in 1440 C.E. wrote of six fortifications before reaching the gates of the royal palace. The areas between the first and second and between the second and third fortification was large and contained agricultural fields, gardens and many residences. From the notes of Robert Sewell it becomes clear that between the third fortification up to the actual fortress, one came across countless people, shops, bazaars. From their accounts the area of the greater metropolitan area of Vijayanagara was about 540 km² which is about 25 times larger than the area comprising the main administrative, sacred and royal centers. The tourist zone itself is limited to the inner urban core.
https://en.wikipedia.org/wiki/Vijayanagara_metropolitan_area
If the greater metropolitan area of Vijayanagara was about 540 km² or 208.495 square miles, and it was surrounded by the outermost walls, and if it was mostly uninhabited after the massacre of 1565, someone could have bricked up most of the outer gates, leaving only a few gates open for entering or leaving the ruined city.
4) Possibly a (possibly abandoned) fortress. Ranikot Fort in Pakistan is believed to be the largest fortress in the world and encloses mostly empty desert. The gates would be the easiest ways in or out and they could be closed.
https://en.wikipedia.org/wiki/Ranikot_Fort
5) A rectangular plot surrounded by canals. From the description of Atlantis in Critias:
I have described the city and the environs of the ancient palace
nearly in the words of Solon, and now I must endeavour to represent
the nature and arrangement of the rest of the land. The whole
country was said by him to be very lofty and precipitous on the side
of the sea, but the country immediately about and surrounding the city
was a level plain, itself surrounded by mountains which descended
towards the sea; it was smooth and even, and of an oblong shape,
extending in one direction three thousand stadia, but across the
centre inland it was two thousand stadia. This part of the island
looked towards the south, and was sheltered from the north. The
surrounding mountains were celebrated for their number and size and
beauty, far beyond any which still exist, having in them also many
wealthy villages of country folk, and rivers, and lakes, and meadows
supplying food enough for every animal, wild or tame, and much wood of
various sorts, abundant for each and every kind of work.
I will now describe the plain, as it was fashioned by nature and
by the labours of many generations of kings through long ages. It
was for the most part rectangular and oblong, and where falling out of
the straight line followed the circular ditch. The depth, and width,
and length of this ditch were incredible, and gave the impression that
a work of such extent, in addition to so many others, could never have
been artificial. Nevertheless I must say what I was told. It was
excavated to the depth of a hundred, feet, and its breadth was a
stadium everywhere; it was carried round the whole of the plain, and
was ten thousand stadia in length. It received the streams which
came down from the mountains, and winding round the plain and
meeting at the city, was there let off into the sea. Further inland,
likewise, straight canals of a hundred feet in width were cut from
it through the plain, and again let off into the ditch leading to
the sea: these canals were at intervals of a hundred stadia, and by
them they brought down the wood from the mountains to the city, and
conveyed the fruits of the earth in ships, cutting transverse passages
from one canal into another, and to the city. Twice in the year they
gathered the fruits of the earth-in winter having the benefit of the
rains of heaven, and in summer the water which the land supplied by
introducing streams from the canals.
http://www.sacred-texts.com/atl/critias.txt
If your characters don't know how to swim, and don't know how to sail on the canals, or the boats are all docked on the opposite sides of the canals, they can only cross the canals where there are bridges. The squares made by the canals seem to have been 100 stadia or about 9.75 to 12.98 miles on each side. But you could make them different sizes if you wanted.
6) Perhaps a very specific setting. The Great Hedge of India was an internal customs barrier in the 19th century. By 1878 the great hedge had 411.75 miles of perfect hedge and 1109.5 miles of inferior hedge, or dead and dry hedge, or stone walls.
There were guard houses spaced along the hedge and gates on the roads with customs houses to collect the tax.
Imagine a meandering river that loops up to your fictional version of the hedge at two places and loops away from it in between. If the river is wide, or swift, or rough enough, there might be very few places to ford it, and few bridges. And there could be few gates in the hedge on the other side.
The segment of hedge could be replaced by a segment of Great-wall-of-China-like defenses if you want. But either way the inhabitants of the loop of land would be left on the outside looking in.
This might have been suggested by Buckland in LOTR, between the Brandywine River in the west and the High Hay keeping out the Old Forest in the east.
And there is the great wall on Skull island in King Kong protecting the village on a peninsula from the dinosaurs.
Added 05-19-2017.
7) Suppose that there is a river running north and south through the No Man's Land Valley between Westlandia and Eastlandia. Westlandia and Eastlandia are often at war, so each has fortified the cliffs on their side of the No Man's Land Valley with walls and a few gates.
Thus most of the fighting between Westlandia and Eastlandia takes place in the flatter ground north and south of the No Man's Land Valley. The No Man's River runs north and south through the No Man's Land Valley but it loops a lot, often hitting the east or west cliffs. Thus the No Man's Land Valley contains many larger or smaller sections of flat land that are surrounded on three sides by the hard to cross No Man's River and on the other side by the east or west cliffs with walls at their tops and guards at the few gates more likely to shoot you with arrows than let you in.
