The eastern section of Seoul was on lower ground than the other sections and was more susceptible to external attack. Thus, a lookout was added to the outside of the gate to reinforce its defense. A part of the walls in the section between Heunginjimun and Gwanghuimun was extended outside in a rectangular shape for such a purpose. Signal fire mounds, another component of the defense system, were first established in 1394 and remained in operation until 1894. Signals sent across the country from one mound to another, using smoke by day and fire at night, were received by the beacon at the top of Namsan and conveyed to the Royal Palace.