Hwaseong has five secret gates for the passage of people, cattle, and military supplies. Seoammun (Western Secret Gate) is a well-hidden passage and also an entrance to Yongdo, a passageway along the outer perimeter of the fortress. Only this gate has a sentry box among the fortress’s five secret gates. Sentries there signaled to other troops for an approaching enemy by waving a flag or firing a gun. Its arched structure (hong-ye) was built in August 6, 1796 (the 20th year of King Jeongjo’s reign). A posa is a sentry post from which, in cases of emergencies such as enemy attack, warning signals through waving a flag or firing a cannon were sent.