A fortification, also known as a fort, fortress, fastness, or stronghold, is a military structure built to protect areas during war and to help control regions during peace. The word "fortification" comes from the Latin words fortis ("strong") and facere ("to make").

From ancient times to today, defensive walls have often been needed to help cities survive during times of invasion and war. Some of the earliest small cities with walls were in the Indus Valley Civilization. In ancient Greece, large stone walls without mortar were built in Mycenaean Greece, such as those at the site of Mycenae. A Greek phrourion was a group of buildings used as a military base, similar to the Roman castellum or fortress. These structures were mainly used as watchtowers to guard roads, mountain passes, and borders. Though smaller than full fortresses, they helped protect borders rather than serve as strong defensive points.

The process of setting up a military camp or building a fortification has been called "castrametation" since the time of the Roman army. Fortifications are usually divided into three types: permanent fortifications, which are long-lasting; field fortifications, which are temporary; and semipermanent fortifications, which are in between. Castles are a type of fortification that are different from regular forts or fortresses because they are homes for kings or nobles and control specific areas.

Roman forts and hill forts were the early models for European castles, which began to appear in the 9th century during the Carolingian Empire. During the Early Middle Ages, some towns were built around castles.

Medieval-style fortifications became less useful in the 14th century when cannons were introduced. Fortifications in the era of black powder weapons became shorter and used more ditches and earth walls to protect against cannon fire. Walls that were directly hit by cannons were easily damaged, so they were built into ditches with sloped earth to improve safety.

The invention of explosive shells in the 19th century changed fortifications again. Star forts, which had complex designs with bastions and firing positions, were not effective against powerful explosives. These designs could be quickly destroyed by shells. Steel and concrete fortifications were widely used in the 19th and early 20th centuries. However, modern warfare since World War I has made large-scale fortifications unnecessary in most situations.

History

Defensive fences were used long before writing was invented. Early humans built these fences to protect themselves and their animals from large predators, such as by blocking cave entrances.

Walls have been important for many cities throughout history. Amnya Fort in western Siberia is one of the oldest known fortified settlements and the northernmost Stone Age fort. In Bulgaria, near the town of Provadia, a walled settlement called Solnitsata, built around 4700 BC, had a diameter of about 300 feet (91 m). It housed 350 people living in two-story homes and was surrounded by a strong wall made of large stone blocks. These blocks were 6 feet (1.8 m) high and 4.5 feet (1.4 m) thick. This settlement is one of the earliest walled places in Europe, though it is younger than the walled town of Sesklo in Greece, built around 6800 BC.

Uruk in ancient Sumer (Mesopotamia) is one of the oldest known walled cities. The Ancient Egyptians built fortresses along the edges of the Nile Valley to protect against invaders and surrounded their cities with circular mud brick walls. Many ancient fortifications were made of mud bricks, which often left only mounds of dirt for archaeologists to study today. A massive prehistoric stone wall, called the "Great Wall of Brodgar," surrounded the ancient temple of Ness of Brodgar in Scotland around 3200 BC. This wall was 4 m (13 ft) thick and 4 m (13 ft) tall and may have had a symbolic or ritual purpose. The Assyrians used large labor forces to build new palaces, temples, and defensive walls.

In Bronze Age Malta, some settlements were fortified. One example is Borġ in-Nadur, where a bastion built around 1500 BC was found. Ancient Sparta and Rome did not use walls for a long time, relying instead on their military for defense. Early fortifications were simple structures made of wood and earth, later replaced by stone walls stacked without mortar. In ancient Greece, large stone walls were built in Mycenaean Greece, such as the ancient site of Mycenae, famous for its "cyclopean" walls made of huge stone blocks. In classical Greece, Athens built two parallel stone walls, called the Long Walls, that connected to its fortified seaport at Piraeus.

In Central Europe, the Celts built large fortified settlements called oppida, whose walls were partly influenced by Mediterranean designs. These fortifications were continuously expanded and improved. Around 600 BC in Heuneburg, Germany, forts were built with a limestone foundation and a mudbrick wall about 4 meters tall, likely topped with a roofed walkway, making the total height 6 meters. The wall was covered with lime plaster, which was regularly renewed. Towers extended outward from the wall.

The Oppidum of Manching, located near modern-day Manching in Bavaria, Germany, was a large Celtic settlement founded in the 3rd century BC and existed until about 50–30 BC. It reached its largest size during the late La Tène period (late 2nd century BC), covering 380 hectares. At that time, 5,000 to 10,000 people lived within its 7.2 km long walls. The oppidum of Bibracte is another example of a Gaulish fortified settlement.

The term "casemate wall" is used in archaeology in Israel and the Near East to describe a double wall surrounding a city or fortress. These walls had transverse walls separating the space between them into chambers, which could be used for storage or living, or filled with soil and rocks during sieges to strengthen the outer wall against battering rams. The earliest known casemate walls date to the 16th century BC at Ti'inik (Taanach). These walls were common in the Southern Levant during the Middle Bronze Age and Iron Age II, peaking in the Iron Age (10th–6th century BC). By the 9th century BC, they were replaced by solid walls, likely due to the development of more powerful battering rams by the Neo-Assyrian Empire. Casemate walls could surround entire settlements or only parts of them. There were three types: freestanding walls, integrated walls where the inner wall was part of the settlement’s buildings, and filled walls where the space between walls was filled with soil immediately.

The Romans built strong, mortar-bound stone walls to fortify their cities. The most famous examples are the Aurelian Walls of Rome and the Theodosian Walls of Constantinople, with partial remains found elsewhere. These walls included city gates, such as the Porta Nigra in Trier and the Newport Arch in Lincoln.

Hadrian’s Wall was built by the Roman Empire across northern England after Emperor Hadrian visited the region in AD 122.

Many forts from the Later Stone Age to the British Raj exist in the Indian subcontinent (modern-day India, Pakistan, Bangladesh, and Nepal). In India, the word "fort" is used for all old fortifications. Evidence of fortifications has been found in many Indus Valley Civilization sites. By about 3500 BC, hundreds of small farming villages dotted the Indus floodplain, many with planned streets and defensive walls. In Kot Diji, stone and mud brick houses were protected by massive stone flood dykes and walls, as neighboring communities often argued over control of farmland. Fortifications varied by site. Dholavira had stone walls, Harappa used baked bricks, Kalibangan had mudbrick walls with bastions, and Lothal had a four-sided layout. Fortifications were also found in Mohenjo-daro. Even small towns, like Kotada Bhadli, showed advanced wall structures, suggesting that nearly all towns in the Indus Valley Civilization were fortified. During the second urbanization period (600–200 BC), forts appeared in the Gangetic Valley, with 15 sites identified, including Kaushambi, Mahasthangarh, Pataliputra, Mathura, Ahichchhatra, Raj

Forts

In modern American use, the word "fort" often describes a place set aside by a government for a military base. These locations usually do not have walls or other defenses, and may include areas like soldier housing, offices, hospitals, or intelligence centers.

Some modern structures called forts are actual fortifications. These are often small and temporary, built by improving existing buildings such as homes or public structures in cities. In open areas, they are made from materials like logs, sandbags, or gabions.

These types of forts are usually used in minor conflicts, such as fights against rebels or very small traditional wars, like the Indonesia–Malaysia confrontation. During this conflict, log forts were used by military units. These forts are not used in major wars because above-ground forts cannot withstand heavy weapons like mortars, RPGs, or powerful small arms.

Prisons and others

Fortifications that are built to keep people inside rather than keep attackers out can also be found in prisons, concentration camps, and other similar places. These topics are explained in other articles, as most prisons and concentration camps are not mainly military forts. However, forts, camps, and garrison towns have sometimes been used as prisons or concentration camps. Examples include Theresienstadt, Guantanamo Bay detention camp, and the Tower of London.