WHAT IS A MOON GATE?

A Moon Gate (Chinese: 月亮门; pinyin: yuèliàng mén) is a circular opening in a garden wall that acts as a pedestrian passageway, and a traditional architectural element in oriental gardens. There is a moon gate in the picture above.  Such a circular entrance symbolises the element earth.  It represents an auspicious balance of Yin and Yang.
Sources:
http://www.amethystlake.co.nz/wawcs0124206/feng_shui.html
http://en.wikipedia.org/wiki/Moon_gate