The Third Dynasty of Ur constitutes the second great empire in the history of humankind, after the reign of Sargon and his successors. It lasted about a century, from around 2112 to 2004 BC, dominating the southern portion of Mesopotamia, where it left behind, among other relics, the great ziggurat of Ur (in a simpler time, I thought “ziggurat of Ur” was the title of a person, not the name of a thing).
Ur's end was sudden. The internal machinery seems to have been wound too tightly. Also, anticipating a pattern repeated in so many later empires, it became the focus of attention from outsiders—here Amorite nomads, evidently tougher and more resilient than the empire itself, and covetous of the empire's wealth.
Ur's response was not entirely passive, however. The authorities did take measures to fend off the incursions of the nomads. What sort of measures? Why, they built a wall.