Great Wall of China
The Great Wall of China (长城 Chángchéng) stretches westward across the provinces and municipalities of Liaoning, Hebei, Tianjin, Beijing, Inner Mongolia Autonomous Region, Shanxi, Shaanxi, and Ningxia Autonomous Region to Gansu in the west.
The Great Wall of China can be visited at many places along its length of several thousand kilometers. Its condition ranges from excellent to ruined, and ease of accessibility varies straightforward to quite difficult. Note that different sections also each have their own admission fees, e.g. if you want to hike from Jinshaling to Simatai then you probably have to pay twice.
The Great Wall, as we know it, is actually a series of several walls built at different times by different emperors. The categories below are non-exhaustive, and all refer to wall systems rather than single monolithic walls.
First Great Wall
First Great Wall was ordered built in 214 BC by Qin Shih Huangti after he had finished consolidating his rule and creating a unified China for the first time. The wall was designed to stop raids by the Xiongnu raiders from the north. 500,000 laborers were used during the 32-year building period to create the First Great Wall.
Although the wall worked at keeping out enemies, it did nothing to stop internal pressures which lead to a regime change in 206 BC and the new leadership of the Han Dynasty. The first Han emperor, Gaozu, was quick to see the benefits of the wall against the raiders and ordered more wall to stretch out as far as Zhaoxiang, Gansu.
Second Great Wall
Over 70 years later, the Han Dynasty were still fighting the raiders since the Great Wall had been left to deteriorate and the raiders had breached it in several places. In 130 BC, Emperor Wudi of the Han Dynasty embarked on a program of extending, rebuilding and fortifying the original First Great Wall. After the emperor finished adding more regions under his rule in 127 BC, he ordered a major expansion program that created the Second Great Wall, outposts in Zhangye, Wuwei, Jiuquan, Dunhuang and Yumenguan in Gansu and Lopnor and other outposts in Xinjiang. The Great Wall was extended down the Hexi Corridor through which the Silk Road traders would travel on the way to and from the West.
When the Han Dynasty fell apart into the three kingdoms of the Wei, Shu and Wu, the northern Wei kingdom decided to continue maintaining the Great Wall so that they could keep out the Rouran and Qidan nomads from the northern plains. Despite the constant maintenance, the Wall kept being breached by the Rouran nomads. Additional walls were built inside and outside of the Great Wall by the different kingdoms. Eventually the Wei kingdom merged with the unifying Sui kingdom and was overthrown by the Tang Dynasty in 618 AD.
Nothing more was done to the Great Wall until the reign of the Liao and Song dynasties. The Liao Dynasty controlled the north while the Song Dynasty controlled the south. The Liao were troubled mainly by a tribe in the northeast region of China called the Nuzhen (known as Manchu in Mandarin) so they built defensive walls along the Heilong and Songhua rivers. These failed to stop the raiders from coming south.
Third Great Wall
In 1115, the Nuzhen established the Jin Dynasty and since they were from the north themselves, understood that the Mongols were right behind them. The Jin emperor ordered the construction of a Third Great Wall to be built in Heilongjiang Province and Inner Mongolia Autonomous Region. The walls built had the characteristics of having ditches running along the walls full length.
Despite the impressive fortifications built, the Mongols overthrew the Jin in 1276 and established the Yuan Dynasty. During the Yuan dynasties rule, the Wall fell into deep disrepair and in 1368, the Chinese Ming Dynasty walked right in and took control.
The Ming Dynasty, after getting rid of the Mongols, determined that they would never be taken again by outsiders. The first emperor of the Ming Dynasty, Hongwu, re-established manning of the Great Wall, fortresses and garrisons were built along the wall, and the fort at Jiayuguan was built in 1372 at the western end of the wall. The second Ming emperor, Yongle, turned his focus outward from the empire and sent out explorers and diplomats into the big, wide world.
Fourth Great Wall
It was not until the battle of Tumu against the Mongols that renewed interest in reinforcing the Great Wall occurred. Between 1569 and 1583, the most well-known parts of the Great Wall were built, the Fourth Great Wall. The reinforced wall managed to repel Mongols several times.
The Manchu retook China in 1644 and formed the Qing Dynasty. From this point on, the Wall slowly started to fade away while stone and rocks were taken from the Wall for building projects and homes. The Cultural Revolution definitely took its toll out on the wall when local people and local governments were encouraged to help dismantle the Great Wall.
It was not until 1984 that President Deng Xiaoping started a restoration and protection project of the Great Wall. In 1987, the Great Wall was declared a World Cultural Heritage Site by UNESCO.
The geography of Northern China ranges from mountainous in northeast Liaoning and Hebei provinces, through the grasslands of Ningxia, semi-arid desert of China's loess plateau, and borders the sand dunes of the Tengger desert of Inner Mongolia. It is the area around Hebei and Beijing that most people associate with the Great Wall, but most of the Great Wall lies in the desert regions of the country.
Flora and fauna
Chinese wildlife is diverse, considering all of the different habitats available along the length of the Great Wall. From the rare Siberian tiger in the northeast to the protected and rare Giant Panda which lives in southern Gansu, Sichuan, and Shaanxi, you never know what you might see on a given day.
Wild mammals can be found in the north, such as the Manchurian weasel, brown and black bears, northern pika, and mandarin vole. Deer species include Sitka deer, roe deer and the long-sought-after spotted deer, which has many uses in Chinese medicine.
The birds of the region include various pheasants, black grouse, pine grosbeak, various woodpeckers, mandarin duck, and the fairy pitta, a rare migratory bird. Cranes are especially revered in China. Common, demoiselle, white-napes, hooded, and red-crowned cranes all breed in China.
You can find many tonic plants along the Great Wall, such as the rare ginseng (Panax ginseng). Chinese medicine has had many thousands of years to discover and use these tonic plants for the benefit of mankind.
Northern China has all four seasons and they arrive with a vengeance. Summer and winter temperatures normally reach extremes of over 40 degrees Celsius (105+ °F) and -20 degrees Celsius (-4 °F) respectively.
Bring a jacket against the wind or cold in the chillier seasons. In summer you will need lots of water, but there are plenty of vendors at the most visited sections. Be prepared for the possibility of sudden, short, but rather violent thunderstorms.
Do not leave any trace of your visit. Even if it is not an uncommon sight, resist the urge to add your name to the carvings in the wall, or take a piece home as a souvenir. If the wall should be damaged by your actions, the authorities may very well take action with fines and other punishments.
Hiking as a recreational sport is not well understood yet in China so the etiquette of crossing state and private land has not yet been established. Remember that the Wall is mostly mud and poorly supported stones, and that you are on your own if you're outside the maintained areas. Even if you are not walking on the wall, you will find few trails to follow and at some parts, the area the Wall traverses are vertical, treacherous and very unsafe. Besides that, it is difficult to obtain clean drinking water and some areas may even have no water at all. Other areas will have man made obstacles, like roads and motorways that have solid fencing. Villages where you could get supplies may be few and far between. Some may take you miles away from the Wall. Poor cartography is still a problem here since maps of less than 1:450,000 are not easy to get a hold of due to the military applications of such maps. Besides that, guides who know the areas along the Great Wall are few and far between. The last item to think about regarding hiking the Great Wall is that China has no system of mountain/wilderness rescue personnel. You will be on your own should something happen to you.
Scams - Beware of bus scams that may ruin your day. Also try to avoid organized tours to the Great Wall costing 100-150 Yuan. These are advertised by people handing out flyers around the Forbidden City in Beijing  for example (the real bus service to the Great Wall only costs 20 Yuan!). Also, the driver might just stop and set you off before your destination.
Walking safely don't run around as you may trip which may result in an injury as the steps are uneven.
Phones use your phone only for taking images, stop tweeting or updating your Facebook status and actually admire the scene for once as you may never get an opportunity like this again (this will also save you battery which you can then use in case of an emergency)
Badaling Take bus 919 back to Beijing (¥12) or the train from either the Badaling or Qinglong Qiao stations (¥14). The last bus 919 leaves at 17:00. There are plenty trains going to Badaling station. Very cheap and super easy from Beijing station.
For other sections hopefully you've come with a tour that is picking you up from that section. Taxis back to Beijing can be quite expensive (even from Badaling, it will probably be over ¥100).
Ming Tombs Many tour operators or private drivers will combine the wall and the Ming Tombs in a day trip. The Ming Tombs are nothing special and are quite plain. Tourists usually skip them unless they are Chinese history buffs.