Seonamammun (Southwestern Secret Gate), is also known as an ammum or secret gate. There are a total of five of these structures at Hwaseong Fortress.
It is inconspicuously located below Seonamposa (Southwestern Sentry Station). The Yongdo (Passageway) leads from here to Seonamgangnu (Southwestern Pavilion). Seonamammun is the only gate which is equipped with a sentry post.
Structures such as these were inconspicuous and hidden out of sight. They were often used by people to bring war munitions and animals into the fortress.
Construction was completed on August 6, 1796.