Frontiers (synonyms: frontier lands; example toponyms: The Outer Edges, The Rim) are the lands surrounding a border of a country and an adjacent wilderness. Frontiers may contain forts, outposts, frontier towns, watchtowers, and trade routes. There may be uncharted rivers. As these lands are typically farther from any central authority, they may attract or foster crime and criminals. As such, they may contain bandit camps or bandit outposts. Frontiers are home to traders, bandits, and rangers.
Frontiers represent the intersection of what is known and what is undiscovered as well as the meeting of civilization with a more primal wilderness. Frontiers can be used to explore these themes. They may contain creatures or peoples long thought extinct or never before classified. People on frontiers may act (or turn into) animals.
When designing frontiers, the terrain itself becomes a central focus. Often frontiers are depicted as forests, but they could be any type of biome or region. With frontiers the land itself is both a danger, and something that can be used to survive. Consider what storms occur, how shelter can be found, what dangers exist and how a person can adapt and live in these regions. The frontier may be a constant struggle to overcome a wilderness.
Imperialistic empires may consider their borders more akin to frontiers to avoid acknowledging the claimed borders or lands of neighboring countries, cultures, or peoples. Expansion into a frontier is easier to justify than expansion into previously claimed territory that is already a home to someone else.