Sanmon

A sanmon (三門 or 山門^?), also called sangedatsumon (三解脱門, gate of the three liberations^?), is the most important gate of a Japanese Zen Buddhist temple, and is part of the Zen shichidō garan, the group of buildings that forms the heart of a Zen Buddhist temple. It can be however often found in temples of other denominations too. Most sanmon are 2- or 3-bay nijūmon (a type of two-storied gate), but the name by itself does not imply any specific architecture.

Its importance notwithstanding, the sanmon is not the first gate of the temple, and in fact it usually stands between the sōmon (outer gate) and the butsuden (lit. "Hall of Buddha", i.e. the main hall). It used to be connected to a portico-like structure called kairō (廻廊^?), which however gradually disappeared during the Muromachi period, being replaced by the sanrō (山廊^?), a small building present on both sides of the gate and containing a stairway to the gate's second story. (Both sanrō are clearly visible in Tōfuku-ji's photo above.)

...
Wikipedia