One of the most interesting gates, used only for castles, is the Yagura-mon, like the one in this photo from the Hiroshima Castle. This kind of gate features a yagura tower built on top and it is used for the castle's main entrances. Besides the great aesthetic value, the tower improves the gate's overall resistance and it makes it a valuable defensive point...
EXIF info:
Nikon D90
Lens: VR 18-55mm F/3.5-5.6G
Focal Length: 36mm
Aperture: F/5.6
Shutter Speed: 1/640s
ISO Sensitivity: ISO 200