Given the question: Information:  - In the history of Europe, the Middle Ages or medieval period lasted from the 5th to the 15th century. It began with the fall of the Western Roman Empire and merged into the Renaissance and the Age of Discovery. The Middle Ages is the middle period of the three traditional divisions of Western history: classical antiquity, the medieval period, and the modern period. The medieval period is itself subdivided into the Early, High, and Late Middle Ages.  - A defensive wall is a fortification used to protect a city, town or other 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, Walls of Benin, 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.  - The Great Wall of China is a series of fortifications made of stone, brick, tamped earth, wood, and other materials, generally built along an east-to-west line across the historical northern borders of China to protect the Chinese states and empires against the raids and invasions of the various nomadic groups of the Eurasian Steppe. Several walls were being built as early as the 7th century BC; these, later joined together and made bigger and stronger, are now collectively referred to as the Great Wall. Especially famous is the wall built 220206 BC by Qin Shi Huang, the first Emperor of China. Little of that wall remains. Since then, the Great Wall has on and off been rebuilt, maintained, and enhanced; the majority of the existing wall is from the Ming Dynasty (13681644).  - A city is a large and permanent human settlement. Although there is no agreement on how a city is distinguished from a town in general English language meanings, many cities have a particular administrative, legal, or historical status based on local law.  - Europe is a continent that comprises the westernmost part of Eurasia. Europe is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, and the Mediterranean Sea to the south. To the east and southeast, Europe is generally considered as separated from Asia by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways of the Turkish Straits. Yet the non-oceanic borders of Europea concept dating back to classical antiquityare arbitrary. The primarily physiographic term "continent" as applied to Europe also incorporates cultural and political elements whose discontinuities are not always reflected by the continent's current overland boundaries.  - The Middle East is a transcontinental region centered on Western Asia and Egypt. The corresponding adjective is "Middle-Eastern" and the derived noun is "Middle-Easterner". The term has come into wider usage as a replacement of the term Near East (as opposed to Far East) beginning in the early 20th century.  - A palace is a grand residence, especially a royal residence, or the home of a head of state or some other high-ranking dignitary, such as a bishop or archbishop.  - A curtain wall is defensive wall between two towers ( bastions ) of a castle , fortress , or town .  - Hadrian's Wall, also called the Roman Wall, Picts' Wall, or "Vallum Hadriani" in Latin, was a defensive fortification in the Roman province of Britannia, begun in 122 AD in the reign of the emperor Hadrian. It ran from the banks of the River Tyne near the North Sea to the Solway Firth on the Irish Sea, and was the northern limit of the Roman Empire, immediately north of which were the lands of the northern Ancient Britons, including the Picts.  - An arrowslit (often also referred to as an arrow loop, loophole or loop hole, and sometimes a balistraria) is a thin vertical aperture in a fortification through which an archer can launch arrows.  - A wall is a structure that defines an area, carries a load, or provides shelter or security. There are many kinds of walls:  - The Atlantic Wall was an extensive system of coastal defence and fortifications built by Nazi Germany between 1942 and 1944 along the coast of continental Europe and Scandinavia as a defence against an anticipated Allied invasion of Nazi-occupied Europe from Great Britain during World War II.  - "Country House" is a song by English alternative rock band Blur. It was released as the lead single from the band's fourth album "The Great Escape" on 14 August 1995. "Country House" was the first of two Blur singles to reach number one on the UK Singles Chart (the second being 1997's "Beetlebum").  - Fortifications are military constructions or buildings designed for the defense of territories in warfare, and also used to solidify rule in a region during peace time. Humans have constructed defensive works for many thousands of years, in a variety of increasingly complex designs. The term is derived from the Latin "fortis" ("strong") and "facere" ("to make").  - A castle (from ) is a type of fortified structure built in Europe and the Middle East during the Middle Ages by nobility. Scholars debate the scope of the word "castle", but usually consider it to be the private fortified residence of a lord or noble. This is distinct from a palace, which is not fortified; from a fortress, which was not always a residence for nobility; and from a fortified settlement, which was a public defence  though there are many similarities among these types of construction. Usage of the term has varied over time and has been applied to structures as diverse as hill forts and country houses. Over the approximately 900 years that castles were built, they took on a great many forms with many different features, although some, such as curtain walls and arrowslits, were commonplace.    After reading the paragraphs above, choose the best answer for the entity that related to 'curtain wall ' with the relationship of 'part of'.  Choices: - 20th century  - 7th century  - archer  - arctic ocean  - arrow  - atlantic wall  - august  - castle  - chinese  - city  - country  - discovery  - english  - eurasia  - europe  - fortification  - great wall of china  - head  - history  - human settlement  - lead  - mediterranean sea  - middle ages  - middle east  - military  - nobility  - north sea  - rock  - series  - settlement  - state  - the middle  - wall  - warfare  - word  - world war ii
The answer is:
fortification