What advantages might a weaker army fighting on home soil have over a stronger invading army?

The army fighting on their home land are more familiar with the area they're fighting and the geological features. Also, weather can play a part, too, for example; In World War II Hitler made an attack on Russia and lost because his army wasn't prepared for the snow and cold weather and thousands died due to frist bite. So, not being used to weather conditions is a disadvantage for the invading army.