A fortification, also called a fort, fortress, fastness, or stronghold, is a military structure built to defend 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 the present, defensive walls have often been needed to help cities survive during invasions and battles. Some of the earliest fortified cities were found in the Indus Valley Civilization. In ancient Greece, large stone walls without mortar were built in Mycenaean Greece, such as 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 mainly acted as watchtowers to guard roads, mountain passes, and borders. Though smaller than true fortresses, they helped protect borders rather than serve as strongpoints.

The practice of building military camps or fortifications has been called "castrametation" since the time of the Roman legions. Fortification is usually divided into three types: permanent, field, and semipermanent. Castles are a type of fortification that are different from general forts or fortresses because they are homes for monarchs or nobles and control specific areas.

Roman forts and hill forts were the main ancestors of European castles, which began appearing in the 9th century during the Carolingian Empire. In the Early Middle Ages, some towns were built around castles.

Medieval-style fortifications became less useful after cannons appeared in the 14th century. Fortifications during the black powder era used lower walls, ditches, and earth ramparts to absorb cannon fire. Walls that faced cannons were weak, so they were built into ditches with sloped earth for better protection.

The invention of explosive shells in the 19th century changed fortifications again. Star forts did not work well against high explosives, and the complex designs of bastions and cannon positions were easily damaged by shells. Steel and concrete fortifications were common in the 19th and early 20th centuries. Advances in warfare since World War I have made large-scale fortifications unnecessary in most situations.

History

Defensive fences were used long ago to protect people and animals from dangerous animals. These fences may have started when early humans blocked cave entrances to stay safe from large predators.

Walls have been important for cities throughout history. In western Siberia, a place called Amnya Fort is one of the oldest known fortified settlements and the northernmost Stone Age fort. In Bulgaria, a settlement named Solnitsata, built around 4700 BC, had walls about 300 feet wide. It had 350 people living in two-story homes and was surrounded by thick stone walls. These walls were taller and stronger than those in some older settlements, like Sesklo in Greece, which dates back to 6800 BC.

Uruk in ancient Sumer (now Iraq) is one of the oldest walled cities in the world. Ancient Egyptians built walls around their cities and built fortresses to protect against enemies. Many ancient walls were made of mud bricks, which over time became just mounds of dirt for archaeologists to find. A large stone wall, called the "Great Wall of Brodgar," surrounded a temple in Scotland around 3200 BC. It was 13 feet thick and tall and may have had a special meaning. The Assyrians used many workers to build palaces, temples, and walls.

In Bronze Age Malta, some settlements had walls. One example is Borġ in-Nadur, where a wall built around 1500 BC was found. Some ancient cities, like Sparta and Rome, did not have walls for a long time, relying instead on their armies for protection. Early walls were made of wood and dirt, later replaced by stone walls stacked without mortar. In ancient Greece, large stone walls were built in Mycenaean times, such as the "cyclopean" walls of Mycenae. In classical Greece, Athens built two stone walls called the Long Walls to connect to its seaport, Piraeus.

In Central Europe, the Celts built large settlements called oppida with walls that looked similar to those in the Mediterranean. These walls were often improved over time. In Germany, a fort built around 600 BC had a limestone base and a mudbrick wall about 13 feet tall. It may have had a roofed walkway on top. The wall was covered with lime plaster, which was regularly replaced. Towers were built outward from the wall.

The Oppidum of Manching in Germany was a large Celtic settlement that existed from the 3rd century BC until about 50–30 BC. At its largest, it covered 380 hectares and had walls 7.2 kilometers long. About 5,000 to 10,000 people lived there. Another example is the oppidum of Bibracte in Gaul.

Casemate walls are a type of double wall used in ancient Israel and the Near East. These walls had spaces between them that could be used for storage, living, or filled with soil and rocks during attacks to strengthen the outer wall. The earliest known casemate walls were found at Ti'inik (Taanach) around 1600 BC. These walls were common in the Southern Levant during the Bronze and Iron Ages but were later replaced by stronger solid walls.

The Romans built strong walls made of stone held together with mortar. Famous examples include the Aurelian Walls of Rome and the Theodosian Walls of Constantinople. Other remains, like the Porta Nigra in Trier, still exist.

Hadrian's Wall was built by the Roman Empire across northern England in AD 122.

In the Indian subcontinent, many ancient forts and walls have been found, dating from the Later Stone Age to the British Raj. The Indus Valley Civilization had many fortified settlements, such as Dholavira, Harappa, and Mohenjo-daro. These cities had planned streets and walls made of stone, mud bricks, or baked bricks. Later, during the second urbanization period (600–200 BC), forts appeared in cities like Kaushambi and Pataliputra.

India has over 180 forts today, with Maharashtra having more than 70. Many were built by Shivaji, founder of the Maratha Empire. Major forts include the Red Fort in Delhi, the Red Fort in Agra, Chittor Fort, Mehrangarh Fort, and Gwalior Fort.

Forts

In modern American usage, forts often describe areas set aside by governments for military purposes. These areas usually do not include actual walls or barriers but may include buildings like soldier housing, offices, medical centers, or intelligence workspaces.

Some modern forts are built as temporary or semi-permanent structures. In city fighting, these forts are made by improving existing buildings, such as homes or public structures. In open-field battles, they are often built using materials like logs, sandbags, or wire cages filled with rocks.

These types of forts are typically used in small-scale conflicts, such as fighting against rebels or minor battles, like the Indonesia–Malaysia confrontation. During this conflict, log forts were used by military groups stationed near the front lines. These forts are not used in major wars because forts on the ground cannot withstand modern weapons like mortars, rocket-propelled grenades, or regular firearms.

Prisons and others

Some barriers are built to keep people inside a place instead of keeping outsiders away. These types of barriers can be found in prisons, concentration camps, and similar locations. Information about these barriers is discussed in other articles because most prisons and concentration camps are not mainly used as military forts. However, some forts, camps, and garrison towns have been used as prisons or concentration camps. Examples include Theresienstadt, Guantanamo Bay detention camp, and the Tower of London.