In playing Civ I noticed that with the exception of ocean, and the rare chokepoint, natural barriers almost never appear. Mountains, deserts, jungles - a bit slower to explore but in the past, it was normal to have entire armies, let alone exploring expeditions perish while trying to get through these. Rivers were natural defensive barriers e.g. the Yangtze River in China was Southern China's greatest natural defense. Countless times in Chinese history, Chinese in the south, though outpopulated, out-gunned and out-numbered were able to maintain successful defense for decades by controlling the Yangtze river with their navies. If you heavily fortify mountains it is harder to get past but you never really replicate situations in the past where you have a huge mountain range with only a few well-guarded passes. Also in the past mountain ranges with winding narrow tracks and dangerous terrain also made it difficult to supply land based armies.