Iljumun is a kind of building functioned as a gate to indicate a Buddhist

Buddhism

Buddhism is a religion and philosophy encompassing a variety of traditions, beliefs and practices, largely based on teachings attributed to Siddhartha Gautama, commonly known as the Buddha . The Buddha lived and taught in the northeastern Indian subcontinent some time between the 6th and 4th...

 site in Korea. It is the boundary between the Buddhist temple and human's worldly life. The gate symbolizes the purification and one must left all of their worldly desires before entering the temple. It is usually found at the entrances to many Buddhist temples and monasteries

Monastery

Monastery denotes the building, or complex of buildings, that houses a room reserved for prayer as well as the domestic quarters and workplace of monastics, whether monks or nuns, and whether living in community or alone .Monasteries may vary greatly in size – a small dwelling accommodating only...

 in the mountains.

It literally means ‘one-pillar gate’, since when viewed from the side, there seems to be a single pillar. The oneness is also a metaphor for non-duality (unity) in spirit and heart.