Assyria was originally (in the Middle Bronze Age) a region on the Upper Tigris, named for its original capital, the ancient city of Assur. Later, as a nation and empire that came to control all of the Fertile Crescent, Egypt and much of Anatolia, the term "Assyria proper" referred to roughly the northern half of Mesopotamia (the southern half being Babylonia), with Nineveh as its capital. The Assyrian kings controlled a large kingdom at three different times in history. These are called the "Old" (20th to 15th centuries BC), "Middle" (15th to 10th centuries BC), and "Neo-Assyrian" (911–612 BC) kingdoms, or periods, of which the last is the most well known and best documented. Assyrians invented excavation to undermine city walls, battering rams to knock down gates, as well as the concept of a corps of engineers, who bridged rivers with pontoons or provided soldiers with inflatable skins for swimming.