wall—noun

1. 
   any of various permanent upright constructions having a length much greater than the thickness and presenting a continuous surface except where pierced by doors, windows, etc.: used for shelter, protection, or privacy, or to subdivide interior space, to support floors, roofs, or the like, to retain earth, to fence in an area, etc.
   
2. 
   Usually, walls. A rampart raised for defensive purposes.
   
3. 
   An immaterial or intangible barrier, obstruction, etc., suggesting a wall: a wall of prejudice.
   
4. 
   A wall-like, enclosing part, thing, mass, etc.: a wall of fire; a wall of troops.
   
5. 
   An embankment to prevent flooding, as a levee or sea wall.
   

- Selected definitions from Dictionary.com.

• To keep things in.
• To keep things out.
• To do both of the above.

History

As comedian Eddie Izzard once said, archaeologists always find a 'series of small walls'. Walls have been around as long as there has been society. Society wanted to keep other tribes out, to keep their animals in, and to have shelter from the elements. These early walls may not be found during archaeological digs (having often been made of wood, animal dung and other degradable substances), but some of the techniques can be traced all the way through to walls that are still standing. A very good Entry on the ways that humans have built walls and structures out of their natural surroundings can be found at Earthbuilding - Building Using Natural Materials .

Often you may not be able to see the walls themselves, but you can see the earthworks that were built, to then be enhanced with fences and walls. Blewburton Hill , overlooking the village of Blewbury in the Vale of the White Horse, Oxfordshire, looks like it has rings around it. These earthworks, providing a series of ditches, were able to make walls effectively taller to provide better defence against invaders. All of the original walls have long gone, but the earthworks still remain.

Physical Walls

There are as many types of wall as there are materials to build with.

• 
  Walls that make up your house are likely to be made of bricks. These are cuboid shapes, made of fired clay and tend to be 22cm by 7.5cm by 7.5cm. They are not stacked on top of one another directly, but always offset by half of their length in order to make the wall more stable¹. They are 'glued' together using mortar (made of sand, cement or lime, and water).
  

• Drystone walls are used when mortar is easily washed away by poor weather, or when there was no access to mortar. They allow wind to filter through the wall, making it more likely to stay standing when battered by the elements. It is also easier to fix, quicker to build and longer lasting than a mortar-constructed wall.
  

• 
  The walls of castles were made from two separate walls of cut stone facing each other with a gap in the middle. The gap was then filled with offcuts of the cut stones, pebbles, and the like, with mortar mixed in. This was pretty solid and much cheaper and faster than building a thick layer consisting entirely of cut and shaped stone blocks. This technique was sometimes used on other buildings too - for example, it is still visible at the ruins of St Mary's Abbey in York.
  

• Wattle and daub is the name given to a method of wall construction widely used northern Europe in the medieval period and earlier. The wall (often the wall of a house or building) was begun using pieces of wood to form a frame. Between these, a sort of woven panel was made using smaller sticks. This is the wattle. A mixture of mud or manure and straw or similar - the straw was important to bind the mud together - was made and spread over the wattle on both sides. This is the daub.
  

• 
  Paper walls are a part of traditional Japanese buildings. Here thick rice paper is stretched over a wooden frame to provide privacy.
  

• 
  Walls made of metal are usually defensive in nature due to the extremely tough nature of the material. A vault, bars of a cage, or a room containing dangerous equipment are good examples.
  

• 
  Climbing walls are artificial surfaces designed for climbers to practice their skills on. They feature a bewildering array of hand- and foot-holds and simulate as closely as possible conditions on real rock. They have proved popular especially in countries where weather conditions are variable.
  

• 
  A 'fire wall' can mean either a program designed to block unwanted intrusions into your computer via the internet or other network, or a literal 'wall of fire'. It can also be the 'wall' between the engine compartment and the cab in a vehicle, or the wall between a house and an attached garage.
  

Metaphysical Walls

An immaterial or intangible barrier, obstruction, etc., suggesting a wall: [for example] a wall of prejudice.

Again, these can come into many forms. These immaterial walls tend to lean towards the conceptual.

• 
  Mental Block (such as Writer's Block ). When there feels like a barrier between you and what you are trying to write, describe or verbalise.
  

• 
  'Chinese walls' is a business term denoting systems and procedures to isolate people making financial decisions from those with inside knowledge that may affect those decisions (ie, a means of preventing illegal 'insider trading'). For example, it is common for senior executives to sit on the boards of more than one company, so it is essential that they do not use inside information gained from one company to influence investment decisions by another company.
  

• 
  Glass Ceiling. A perceived inability for a particular group in society to rise to higher levels, due soley to their belonging to such a group. This term is usually used to describe women who cannot be promoted due to discrimination against their sex, despite this being illegal in the countries where this phrase is commonly used.
  

• 
  Mental Walls. There is no set term for the mental barriers people place between themselves and the world, especially people. Some people place walls between people they don't know, and some people shut out others entirely. This may be due to a conscious decision, or due to episodes in their past tarnishing relationships with people.
  

• 
  A wall can also be used to describe a non-physical structure such as 'a wall of noise'. This is where noise is so overwhelming it feels like an invisible wall is there.
  

Metaphoric Walls

Walls appear in many metaphors. Here are a few, though this list is not intended to be exhaustive:

First Against the Wall

This indicates a person who is so disliked that they would be selected to be the first to face a firing squad. Douglas Adams gives an example of this usage in The Hitchhiker's Guide to the Galaxy:

The Hitchhiker's Guide to the Galaxy defines the marketing division of the Sirius Cybernetics Corporation as 'a bunch of mindless jerks who'll be the first against the wall when the revolution comes,' with a footnote to the effect that the editors would welcome applications from anyone interested in taking over the post of robotics correspondent.

Curiously enough, an edition of the Encyclopaedia Galactica that had the good fortune to fall through a time warp from a thousand years in the future defined the marketing division of the Sirius Cybernetics Corporation as 'a bunch of mindless jerks who were the first against the wall when the revolution came.'

Writing Is On the Wall²

This doesn't mean that someone has sprayed graffiti on your wall, though this is a popular use of walls in the modern age. It is used to mean that the end of something is inevitable, such as 'from the moment the big supermarket moved in, the writing was on the wall for the village shop'.

The pop group Destiny's Child clearly had their own interpretation in mind on their 1999 track Intro (The Writing's On The Wall), when they wrote:

It's obvious, the writing's on the wall, thou shalt not hate.

Wall of Silence

This term is more commonly used in certain types of police investigation where a neighbourhood or group of people maintain a silence so deep that it is described as being a wall. This barrier often make the difference between solving the crime, and not doing so. For example, after the murder of schoolboy Jessie James in Manchester, BBC News noted that:

Jessie James was shot dead in a community that has had to get used to gun culture and the so-called wall of silence that comes with it.

The Wall; Hitting the Wall

Athletes, particularly long-distance runners, talk of the dreaded Wall. This is the point at which a runner feels utterly unable to go further and is physically and mentally spent. Experts from the English Institute of Sport explained on BBC Athletics:

Setting out at too high a pace will result in you running out of carbohydrate stores and 'hitting the wall' prematurely.

Going To the Wall

This phrase originally comes from medieval times, when attending church was obligatory and before pews were common. The saying went 'the weak go to the wall', because the elderly and infirm were allowed to rest against the walls for support. The term is now used most frequently in business to describe a business that is on the verge of bankrupcy. In the Times, the chief executive of Dixons said of a crisis in the high street:

Downturns in markets make life tough and a bit difficult, but the strong get stronger and the weak get weaker or go to the wall.

Off the Wall

The phrase 'off the wall' is used to describe anything extremely unusual or bizarre. The origins of the phrase are unclear, although some speculate that it hails from ball games such as squash, where the direction of the ball after it bounces off the wall is often extremely unpredictable.

The Beeblecast prepared by the h2g2 Aviators is a pretty off the wall affair, and offers a diverse range of clips for your viewing pleasure.

Famous Walls

There are all sorts of walls that deserve a mention.

The Great Wall of China

Great Wall of China is the longest wall ever built, and can be viewed from outer space. It was built to stop Mongols from invading the South part of the Chinese Empire.

Jericho

Jericho is possibly the first recorded walled city ever. The first mention made of it is found in Numbers 22:1 of the Bible:

Then the Israelites travelled to the plains of Moab and camped along the Jordan across from Jericho.

The only evidence that it had a wall, which seemed to be sort of metaphoric, states:

By faith the walls of Jericho fell, after the people had marched around them for seven days. -Hebrews, 11:30

Perhaps this was not a literal wall? Perhaps it is a mistranslation of the word 'wall'? Perhaps it meant a barrier of some other kind. More research needed.

The Berlin Wall

The Berlin Wall was erected in August 1961 to prevent people from leaving East Germany for the better economy of West Germany. The wall bisected the city without regard to streets or train lines, even utility lines were cut off. The Wall was heavily guarded to prevent attempts to scale it, and more than 200 people died in the attempt over the course of the Wall's history.

In late 1989 the Berlin Wall began to come down, paving the way for the reunification of Germany. By 1991 only a few sections of the Wall remained, and the path where it stood is now marked by paving stones for remembrance.

Hadrian's Wall

A Roman defensive wall started around 122 AD, Hadrian's Wall is located in northern Britain and runs between the river Tyne and the Solway Firth. It was built to protect the 'civilised' south of Pax Britannica from the marauding tribes in the north, and was originally up to six metres tall and about as wide. Milecastles were built every Roman mile, and in between two turrets were posted every third of a mile. It is now a UNESCO Word Heritage Site.

In brief the route travels from Bowness on Solway to Drumburgh, Burgh by Sands and on to Stanwix where it crosses to the southern bank of the River Eden. The road then goes on to Castlesteads then to Birdoswald and then to Great Chesters. It then continues to Housesteads, Carrawburgh and Chesters where it crosses the North Tyne River. The wall continues to Halton Chesters, Rudchester, Benwell Hill and Newcastle upon Tyne and a bridge over the River Tyne. The last two forts are on the banks of the River Tyne, the first at Wallsend on the north bank, and the second at South Shields on the south bank at the mouth of the river, a distance of 74 miles.

The Antonine Wall

When Emperor Antoninus Pius succeed Hadrian in 136 AD, he ordered a new wall built 98 miles to the north of Hadrian's wall. The Antonine Wall as it was called was started in 142 AD, and it ran from the Firth of Clyde to Bo'ness, Falkirk, on the Firth of Forth.

It was built of earth reinforced with timber and stone, with a road to the south, and was protected by a ditch to the north. This was not as well built as Hadrian's Wall, but still impressive and an effective fortification, with a fort approximately every two miles.

Irishman's Wall

In the bleak north of Dartmoor is a kilometre-long wall known as Irishman's Wall. According to local folklore, the wall was erected back in the days when you could claim land in the wilds by simply enclosing it with a wall. An ambitious local landowner decided to claim some land close to the village of Okehampton, and paid a team of strong Irish workers to get the job done quickly. The villagers, unhappy at losing common land but not bold enough to take the Irishmen on face-to-face, decided sabotage was the safest solution. Night after night, they went out and knocked over sections of the wall until eventually the Irishmen and their master grew tired of the charade and called the project off. The wall remains incomplete to this day.

A Few Other Walls

Pink Floyd's 'The Wall'

The Pink Floyd album The Wall was about a man who had an inability to socialise with others without panicking. This disability was in the metaphorical form of a Wall between him and everyone else. There was later a film made using the music of the album. The film contained no dialogue and instead used the images of the music, along with artwork by Gerald Scarfe to convey the storyline.

Wall Street

Wall Street is the financial district of the United States, but originally there really was a wall. It was built by settlers in 1644 to protect the New Amsterdam settlement from Native American and British attack. The street ran alongside the wall and, when the wall was taken down in 1699 by the British, the street remained.

On 17 May, 1792, the Buttonwood Agreement was signed under a buttonwood tree on Wall Street creating an organisation of stock brokers that would, on 8 March, 1817, rename themselves the New York Stock and Exchange Board, headquartered at 40 Wall Street. Today the New York Stock Exchange is one of the largest in the world and is one of several stock exchanges located on Wall Street.

And Finally

Finally, one last wall from the realm of mythology: in a Roman romance, a young man named Pyramus falls in love with the girl next door, whose name is Thisbe. Sadly, the parents of Pyramus and Thisbe are not the best of friends, and forbid them from seeing one another. The couple are thus only able to signal their love from afar, until they discover a small hole in the wall which seperates their houses, whereupon their love blossoms further, and a plan is made to run away and live happily ever after. Unfortunately, the plan falls through, and everyone (apart from the wall) dies.

This tragic story has been re-told in many different guises. Some may already have recognised Romeo and Juliet, but Shakespeare in fact tells the tale twice. In A Midsummer Night's Dream, a troupe of amateur actors, known as the Rude Mechanicals, rehearse and perform this romance for the entertainment of the lead characters³. So far, so inconsequential. However, during their rehearsal, the Mechanicals stumble upon a stage management problem, as expressed by Snout, and a solution is found, given by Bottom. Let their words stand as words of wisdom, or even practical advice, to anyone wishing to erect any form of barrier, be it metaphysical or physical, between people or places; if more heed were taken, perhaps the world would be a more peaceful place.

Snout:
You can never bring in a wall. What say you, Bottom?


Bottom:
Some man or other must present Wall: and let him have some plaster, or some loam, or some rough-cast about him, to signify wall; and let him hold his fingers thus, and through that cranny shall Pyramus and Thisby whisper.