Which feature is considered a natural barrier

When against water it is trees are a natural barrier 

An example would be the Great Wall of China, believe it or not.

So  if  the sum of the enthalpies of the reactants is greater than the products, the  reaction  will be  exothermic.  if  the products side has a larger enthalpy, the  reaction  is  endothermic. you may wonder why  endothermic reactions, which soak up energy or enthalpy from the environment, even happen

Answer (true);