In the form school of feng shui, we learn to choose a site according to the surrounding landscape - hills and mountains: that is the concept of the four celestial guardians (see also the chapter Feng Shui). One of the animals, the black tortoise, represents a mountain which should be in the back, that is behind the house.
Mountains can be replaced by buildings
If you live in a city and do not have mountains nearby, buildings fit the purpose as well. In this case, you have to consider the height of the buildings: a big building represents a big mountain, a little house rather a small hill.
The importance of mountains in feng shui
A mountain signifies support and protection for the entire household. But it must be placed behind the house to have a beneficial effect, if it is placed in front of the entrance door, it will attract obstacles and problems. You can imagine a mountain like a chair back: it is only useful behind you.
A mountain attracts also support and success in the office: try to arrange your desk in a way to have the mountain behind you and you will see that your career will boom rapidly.
If you have neither mountains, nor buildings behind you, a painting with mountains will have the same result.