What is "souimun"

Souimun (Hangul 소의문, Hanja 昭義門; also known as Southwest Gate) was one of the Eight Gates of Seoul in the Fortress Wall of Seoul, South Korea, which surrounded the city in the Joseon Dynasty. The gate was also known as Seosomun (서소문, “West Small Gate”). The gate no longer exists, and there is a marker placed roughly where the gate once stood.