It can be upgraded at the Granary once Bronze Age is reached and can be further upgraded during the Iron Age into the Fortified Wall. Like all buildings, when Architecture is researched, its hit points increases (totaling up to 360) and build time is reduced.
One of the earliest human settlements yet discovered is the city of Jericho near the Jordan River in modern Israel. This site from 7000 BC is remarkable for possessing a stone masonry wall, dry moat around the wall, and a tower. At an astonishingly early age, Jericho demonstrates that the ancients understood principles of fortification that would carry forward essentially unchanged until the development of gunpowder. The Medium Wall is a defensive structure built of stone or other substantial construction to withstand a protracted attack.