The Great Wall of China is the collective name of a series of fortification systems generally built across the historical northern borders of China to protect and consolidate territories of Chinese states and empires against various nomadic groups of the steppe and their polities. Several walls were being built from as early as the 7th century BC by ancient Chinese states; selective stretches were later joined together by Qin Shi Huang (220–206 BC), the first Emperor of China. Little of the Qin wall remains. Later on, many successive dynasties have built and maintained multiple stretches of border walls. The most well-known sections of the wall were built by the Ming dynasty (1368–1644). Apart from defense, other purposes of the Great Wall have included border controls, allowing the imposition of duties on goods transported along the Silk Road, regulation or encouragement of trade and the control of immigration and emigration. Furthermore, the defensive characteristics of the Great Wall were enhanced by the construction of watch towers, troop barracks, garrison stations, signaling capabilities through the means of smoke or fire, and the fact that the path of the Great Wall also served as a transportation corridor. The frontier walls built by different dynasties have multiple courses. Collectively, they stretch from Liaodong in the east to Lop Lake in the west, from the present-day Sino–Russian border in the north to Taohe River in the south; along an arc that roughly delineates the edge of Mongolian steppe. A comprehensive archaeological survey, using advanced technologies, has concluded that the walls built by the Ming dynasty measure . Another archaeological survey found that the entire wall with all of its branches measures out to be . Today, the defensive system of the Great Wall is generally recognized as one of the most impressive architectural feats in history.

Madrid is the capital and most populous city of Spain. The city has almost 3.3 million inhabitants and a metropolitan area population of approximately 6.5 million. It is the third-largest city in the European Union (EU), surpassed only by London and Berlin, and its monocentric metropolitan area is the third-largest in the EU, smaller only than those of London and Paris. The municipality covers . Madrid lies on the River Manzanares in the centre of both the country and the Community of Madrid (which comprises the city of Madrid, its conurbation and extended suburbs and villages); this community is bordered by the autonomous communities of Castile and León and Castile-La Mancha. As the capital city of Spain, seat of government, and residence of the Spanish monarch, Madrid is also the political, economic and cultural centre of the country. The current mayor is José Luis Martínez-Almeida from the People's Party. The Madrid urban agglomeration has the third-largest GDP in the European Union and its influence in politics, education, entertainment, environment, media, fashion, science, culture, and the arts all contribute to its status as one of the world's major global cities. Madrid is home to two world-famous football clubs, Real Madrid and Atlético Madrid. Due to its economic output, high standard of living, and market size, Madrid is considered the major financial centre and the leading economic hub of the Iberian Peninsula and of Southern Europe. It hosts the head offices of the vast majority of major Spanish companies, such as Telefónica, IAG or Repsol. Madrid is also the 10th most liveable city in the world according to Monocle magazine, in its 2017 index. Madrid houses the headquarters of the UN's World Tourism Organization (UNWTO), the Ibero-American General Secretariat (SEGIB), the Organization of Ibero-American States (OEI), and the Public Interest Oversight Board (PIOB). It also hosts major international regulators and promoters of the Spanish language: the Standing Committee of the Association of Spanish Language Academies, headquarters of the Royal Spanish Academy (RAE), the Cervantes Institute and the Foundation of Urgent Spanish (Fundéu BBVA). Madrid organises fairs such as FITUR, ARCO, SIMO TCI and the Madrid Fashion Week. While Madrid possesses modern infrastructure, it has preserved the look and feel of many of its historic neighbourhoods and streets. Its landmarks include the Plaza Mayor, the Royal Palace of Madrid; the Royal Theatre with its restored 1850 Opera House; the Buen Retiro Park, founded in 1631; the 19th-century National Library building (founded in 1712) containing some of Spain's historical archives; many national museums, and the Golden Triangle of Art, located along the Paseo del Prado and comprising three art museums: Prado Museum, the Reina Sofía Museum, a museum of modern art, and the Thyssen-Bornemisza Museum, which complements the holdings of the other two museums. Cibeles Palace and Fountain have become one of the monument symbols of the city.