Use of Terrain
Terrain can be used as a force multiplier by the smaller force and as a force inhibitor against the larger force. Such terrain is called difficult terrain.
The contour of the land is an aid to the army; sizing up opponents to determine victory, assessing dangers and distance. "Those who do battle without knowing these will lose." ― Sun Tzu, The Art of War
The guerrillas must move amongst the people as a fish swims in the sea. ― Mao Zedong.
A good example of this type of strategy is the Battle of Thermopylae, where the narrow terrain of a defile was used to funnel the Persian forces, who were numerically superior, to a point where they could not use their size as an advantage.
For a detailed description of the advantages for the weaker force in the use of built-up areas when engaging in asymmetric warfare, see the article on urban warfare.
Read more about this topic: Asymmetric Warfare