As Slavic tribes united to form states, gords were also built for defensive purposes in less-populated border areas. Gords in which rulers resided or that lay on trade routes quickly expanded. Near the gord, or below it in elevation, there formed small communities of servants, merchants, artisans, and others who served the higher-ranked inhabitants of the gord. Each such community was known as a "suburbium" (). Its residents could shelter within the walls of the gord in the event of danger. Eventually the "suburbium" acquired its own fence or wall. In the High Middle Ages, the gord usually evolved into a castle or citadel (kremlin) and the "suburbium" into a town.