Short answers, pulled from the story.
A defensive wall is a fortification usually used to protect a city, town, or other settlement from potential aggressors. They range from simple palisades or earthworks to extensive military fortifications with curtain walls, towers, bastions, and gates. Some walls, such as the Great Wall of China and Hadrian's Wall, extended far beyond a single city to enclose entire regions.
Uruk in ancient Sumer is one of the world's oldest known walled cities. Before that, the proto-city of Jericho in the West Bank had a wall surrounding it as early as the 8th millennium BC. The earliest known town wall in Europe belonged to Solnitsata, built in the 6th or 5th millennium BC.
Chinese walls had tamped earthen cores that absorbed the energy of artillery shots, built inside wooden frameworks filled with earth pounded to a highly compact state. Ming prefectural and provincial capital walls ran 10 to 20 metres thick at the base, while medieval European walls usually stayed around 2 metres. According to Tonio Andrade, this immense thickness held back the development of larger cannon, since even industrial-era artillery struggled to breach it.
The star fort, also known as the bastion fort or trace italienne, was a style of fortification that became popular in Europe during the 16th century. It was developed in Italy, where the Florentine engineer Giuliano da Sangallo, who lived from 1445 to 1516, compiled a defensive plan around the geometric bastion. Its angle bastions supported their neighbors with lethal crossfire covering every angle.
Many city walls were demolished as cities grew and defensive strategy shifted toward forts ringing the city, and because gunpowder made walls less effective once siege cannon could blast through them. In the 19th century, many structures were torn down to modernize cities. Today a vanished wall often shows only as a ditch, ring road, or park.
Modern defensive walls include the Berlin Wall, built around West Berlin by the German Democratic Republic from 1961 to 1989, and the Korean Demilitarized Zone near the 38th parallel north. Others include the Nicosia Wall along the Green Line in Cyprus, the peace lines in Belfast, and the Gaza-Israel barrier first built by Israel in 1971. In September 2014, Ukraine announced a European Rampart along its border with Russia.