The Forbidden City is surrounded by a 7.9-metre high city wall and a six-metre deep, 52-metre wide moat. The walls are 8.62 metres wide at the base, tapering to 6.66 metres at the top. These walls served as both defensive walls and retaining walls for the palace. They were constructed with a rammed earth core, and surfaced with three layers of specially baked bricks on both sides, with the interstices filled with mortar.

At the four corners of the wall sit towers with intricate roofs boasting 72 ridges, reproducing the Pavilion of Prince Teng and the Yellow Crane Pavilion as they appeared in Song Dynasty paintings. These towers are the most visible parts of the palace to commoners outside the walls, and much folklore is attached to them. According to one legend, artisans could not put a corner tower back together after it was dismantled for renovations in the early Qing Dynasty, and it was only rebuilt after the intervention of carpenter-immortal Lu Ban.

The wall is pierced by a gate on each side. At the southern end is the main Meridian Gate. To the north is the Gate of Divine Might, which faces Jingshan Park. The east and west gates are called the "East Glorious Gate" and "West Glorious Gate". All gates in the Forbidden City are decorated with a nine-by-nine array of golden door nails, except for the East Glorious Gate, which has only eight rows.

The Meridian Gate has two protruding wings forming three sides of a square (Wumen, or Meridian Gate, Square) before it. It is also called the "Five Phoenix Pavillion" because the superstructure is composed of five buildings. Imperial proclamations and almanacs are issued from the gate house. After successful campaigns, the Emperor received prisoners of war here, sometimes followed by mass decapitations. The gate has five gateways. The central gateway is part of the Imperial Way, a stone flagged path that forms the central axis of the Forbidden City and the ancient city of Beijing itself, and leads all the way from the Gate of China in the south to Jingshan in the north. Only the Emperor may walk or ride on the Imperial Way, except for the Empress on the occasion of her wedding, and successful students after the Imperial Examination.

Chinese city wall

Chinese city walls (Chinese: 냘퓰; Pinyin: chĲngqiÐng; literally "city wall") refer to civic defensive systems used to protect towns and cities in China in pre-modern times. The system consisted of walls, towers, and gates, which were often built to a uniform standard throughout the Empire.

Defensive wall

A defensive wall is a fortification used to defend a city or settlement from potential aggressors. In ancient to modern times, they were used to enclose settlements. Generally, these are referred to as city walls or town walls, although there were also walls, such as the Great Wall of China, Hadrian's Wall, and the metaphorical Atlantic Wall, which extended far beyond the borders of a city and were used to enclose regions or mark territorial boundaries. Beyond their defensive utility many walls also had important symbolic functions ― representing the status and independence of the communities they embraced.

Walls are almost always masonry structures, although brick and timber-built variants are also known. Depending on the topography of the area surrounding the city or the settlement the wall is intended to protect, elements of the terrain (e.g. rivers or coastlines) may be incorporated in order to make the wall more effective.

Walls may only be crossed by entering the appropriate city gate and are often supplemented with towers. In the Middle Ages, the right of a settlement to build a defensive wall was a privilege, and was usually granted by the so-called "right of fortification". The practice of building these massive walls, though having its origins in prehistory, was refined during the rise of city-states, and energetic wall-building continued into the medieval period and beyond in certain parts of Europe.

Retaining wall

A retaining wall is a structure that holds back soil or rock from a building, structure or area. Retaining walls prevent downslope movement or erosion and provide support for vertical or near-vertical grade changes. Cofferdams and bulkheads, structures that hold back water, are sometimes also considered retaining walls. Retaining walls are generally made of masonry, stone, brick, concrete, vinyl, steel or timber. Once popular as an inexpensive retaining material, railroad ties have fallen out of favor due to environmental concerns. They also decompose over time.

Segmental retaining walls have gained favor over poured-in-place concrete walls or treated-timber walls. They are more economical, easier to install and more environmentally sound.

The most important consideration in proper design and installation of retaining walls is that the retained material is attempting to move forward and downslope due to gravity. This creates a soil pressure behind the wall which depends on the angle of internal friction (phi) and the cohesive strength (c) of the retained material, as well as the direction and magnitude of movement the retaining structure undergoes. Lateral earth pressures are typically smallest at the top of the wall and increase toward the bottom. Earth pressures will push the wall forward or overturn it if not properly addressed. Also, any groundwater behind the wall that is not dissipated by a drainage system causes an additional horizontal hydraulic pressure on the wall.

Rammed earth

Rammed earth construction, also known as pisĲ de terre or simply pisĲ, is an age-old building method that has seen a revival in recent years as people seek low-impact building materials and natural building methods. Traditionally, rammed earth buildings are common in arid regions where wood is in scarce supply.

Rammed earth construction is a process of compressing a damp mixture of earth that has suitable proportions of sand, gravel and clay (sometimes with an added stabilizer) into an external supported frame that molds the shape of a wall section creating a solid wall of earth. Traditional stabilizers such as lime or animal blood were used to stabilise the material, but cement has been the stabilizer of choice for modern times. After compressing the earth the wall frames can be immediately removed and require an extent of warm dry days after construction to dry and harden. The structure can take up to two years to completely cure, and the more it cures the stronger the structure becomes. When the process is complete it is much like constructing a hand made wall of solid rock.

Formwork is set up creating the desired shape of the section of wall, damp material is poured in to a depth of between 100 to 250mm (4 to 10 inches). A pneumatically powered backfill tamper - something like a hand-held pogo stick with a flat plate on the bottom - is then used to compact the material to around 50% of its original height. Further layers of material are added and the process is repeated until the wall has reached the desired height. The wall is so solid that if desired the forms can be removed immediately. This is necessary if wire brushing to reveal texture is desired otherwise walls become too hard to brush after around 60 minutes. Walls take some time to dry out completely, but this does not prevent further work on the project. Any exposed walls should be sealed to prevent water damage - there are several proprietary products specifically designed to seal earth walls.

In modern variations of the method the rammed earth walls are constructed on top of conventional footings or a reinforced concrete base, sometimes with extra ground insulation from a horizontal layer of styrofoam. Some builders also add coloured oxides or other items such as bottles or pieces of timber to add variety to the structure.

Once completely cured the walls are very workable. It is easy to drive a nail or screw into them and they can be patched if necessary with the result being undetectable if the same material was used.

One of the significant benefits of rammed earth constructions is its excellent thermal mass; it heats up slowly during the day and releases its heat during the evening. This can even out daily temperature variations and reduce the need for air conditioning and heating. On the other hand, rammed earth is not a good insulator. Like brick and concrete (which also have excellent thermal mass), rammed earth is often insulated in colder climates. The thickness and density of the walls lends itself naturally to soundproofing and the materials used in the walls make them virtually fireproof.

Prior to the use of cement as a stabilizer, rammed earth buildings were most successful in dry climates with limited availability of building materials other than earth. Rammed earth has become a viable material in wetter climates, either through the use of cement stabilisation, through placing the earth walls within the weatherproof fabric of the building, or by the application of external insulation and weatherproofing.

View a tour plan to Tian'an men Square and Forbidden City