Since times ancient walls have served as fortifications in military defense structures. Early fortifications usually consisted in simple earth ramparts – often complemented with wooden palisades and external ditches. As defense mechanisms became more sophisticated, ramparts began to be built of stone or brick and were endowed with walking platforms, watchtowers, and shooting slits or crenels to more efficiently withstand sieges. With the progress of artillery (cannons) in the 14th century walls became ever thicker and, from the 16th century onwards, they were increasingly endowed with bastions allowing interlocking fire. With the massively improved firepower of artillery and the use of military aircraft, however, the importance of walls declined greatly in contemporary defense strategies.

City walls or town walls have been common since Antiquity and have usually been punctuated with watchtowers and several massive gates. City walls commonly integrated topographic assets such as hillside location and/or proximity to rivers or seas to maximise their defensive potential. During the Middle Ages, city walls were booming and played a crucial role in medieval societies, imaginaries and economies. They served to delimit the political rights of burghers, regulate trade, collect taxes and signal a city's prestige. In the wake of rapid city growth and changing military technology (cannons), city walls progressively fell into obsolescence and were frequently dismantled.

Border barriers are markers of separation running along international borders. They may be composed of different materials such as sand, wood, concrete, metal parts, fencing, barbed wire or, frequently, a combination thereof. Border barriers can range from the porous (e.g., mere berms or fencing) to the seemingly impermeable (e.g., multiple ranges of slabs of concrete equipped with tunnel and movement detectors, drones, night-vision equipment, floodlighting, etc.). They are typically built for a series of differing objectives such as security, controlling migration and discouraging traffickers. In certain cases, border barriers have also been built to buttress de facto claims over disputed international territory.

An enclave is a part of a country that is surrounded by another country. Some enclaves such as the Spanish autonomous cities of Ceuta and Melilla in Northern Africa have extensive border fences and controls. Ceuta and Melilla each have two rows of high fences, completed with barbed wire, watchtowers, a patrol road and noise and movement sensors. Other famous enclaves include Kaliningrad (on whose border Lithuania is currently building a border fence), Nagorno Karabakh (an Armenian enclave surrounded by Azerbaijan), and the countless enclaves – including second-order enclaves – on both sides of the India-Bangladesh border, but for which India and Bangladeshi have signed an exchange agreement in 2015.

Gated communities – barrios privados, condominio fechado or “colonies” – form residential complexes with entry and exit points guarded by private security personnel. They have mushroomed in countries such as India, South Africa, Canada, Brazil, China, India, Mexico and Pakistan, and epitomise new forms of social and/or ethnic apartheid in urban settings. Sometimes gated communities are explicitly designed to cater to expat communities of foreign workers, as is frequently the case in the Middle East. Their perimeter is usually delimited by a circular set of walls and/or fences designed to shield their inhabitants from exterior violence. They are frequently built for self-sustainability (with their own power infrastructure, water reserves, sanitation, fuel, etc...) and in given cases (e.g. sects) also to prevent contact from community members with the outside world.

“Embassy districts” – and sometimes military bases – constitute another form of territorial enclaves. Diplomatic missions have frequently been grouped as gated communities, sometimes with extensive rights of extraterritoriality, as in the case of the 19th century Legation Quarter in Beijing. More recently, Pakistan has created a diplomatic enclave in Islamabad, constituting a special secured zone housing 43 embassies and high commissions which is closed to the general public. There are many other diplomatic enclaves in the world such as those of Kabul or Delhi (Chanakyapuri) but they are not necessarily gated.

Neighbourhood walls usually consist in a series of separation barriers designed to counter inter-communal – often religious – violence within urban settings. The most famous sectarian barriers are the Belfast Peace Lines separating since 1969 the mainly Protestant-Unionist from the mainly Catholic-Nationalist areas. More recent examples include the 5-km-long “Baghdad Wall” built by the United States Army between the predominantly Sunni district of Adhamiya and the surrounding Shiite districts, officially to prevent death squads and suicide bombings; or the cement wall built by the Syrian government to isolate the Sunni neighbourhood and former rebel stronghold of Baba Amr from the more upmarket and mostly Shia and Christian Al-Insha’at neighbourhood.

Booms or defensive chains spanning across harbours or rivers to prevent hostile naval attacks have been common in history. The chains would often be attached to towers allowing to raising or lowering them. Famous examples include the Roman Leonine Wall blocking the Tiber or the Golden Horn Chain stretching from Eminonu to Galata. More modern examples include anti-submarine nets that have been in use since World War I. Israel is currently building a sea barrier to reinforce its maritime delimitation with the Gaza Strip and “prevent infiltrations”. China, for its part, is working on the “Underwater Great Wall Project”, a surveillance system composed of a network of vessels with the aim of consolidatong its maritime ambitions in the Asia Pacific.

The first ghetto was created in medieval Venice in the 16th century. Its roughly 1,000 inhabitants were locked in at night but were free to move and trade during the day. Throughout the 18th and 19th centuries ghettoisation spread, with Jewish families settled by force into cities such as Venice, Frankfurt, Prague and Trieste. The Nazis systematised the ghetto system and closed Jews off hermetically behind walls and barbed wire. At its peak, the Warsaw Ghetto counted more than 400,000 Jews facing disease and starvation. Escapees were shot on sight. Today, Europe's estimated 10–12 million Roma face ghettoisation in several Eastern European countries. In Stolipinovo, Bulgaria, for instance, as many as 50,000 predominantly Roma (Gypsy) people are densely packed into gated communities. Several Slovak municipalities such as Ostrovany have erected walls to separate the residents of Roma ghettos from their white neighbours.

Walls have often been used to delimit agricultural and other parcels of land. Especially in areas of rocky soils, it was common for farmers to pile up the rocks gained from clearing their fields in order to demarcate their property. Traditional stone walls have thus become a regular feature of many rural landscapes.

Walls have further been built to protect human collectivities from natural disasters such as flooding (seawalls, leaves), storms or avalanches. Other protective walls serve to keep out noise from airports or highways. Sometimes, walls are explicitly designed for environmental protection – to preserve rather than to keep out –, as is the case in retaining walls whose objective is to stabilise terrains, prevent soil erosion or collect water.

Even the seemingly borderless digital world is now replete with “gateways”, “access keys”, “block chains” and surveillance mechanism – which is perhaps best illustrated by the “Great Firewall of China”. China's Bureau of Public Information and Network Security Supervision combines legislative actions and new technologies to maintain “cyber sovereignty” and prevent its citizens from accessing “subversive” content. Its “Golden Shield Project” includes the outright blockage of foreign sites such as Google, Facebook and Twitter, DNS poisoning, self-censorship, manual enforcement through a brigade of hundreds of thousands of civil censors and, most importantly, blocking access to VPN services. Famously, China's digital censorship has led to the banishment of Winnie the Pooh, whose physical resemblance to chairman Xi has been deemed unbearable. China is not alone in erecting digital walls, as indicated by the annual reports of Reporters without Borders' singling out on a regular basis “enemies of the internet” such as  Bahrain, Iran, Syria, Vietnam, Saudi Arabia, Cuba, North Korea and Belarus.