한양도성은 조선 왕조 도읍지인 한성부의 도심 경계를 표시하고 그 권위를 드러내며 외부의 침입을 방어하기 위해 축조된 성이다. 흥인지문은 1398년에 축조할 당시 서울 도성의 동쪽 문이었으며 지금의 문은 1869년에 다시 지은 것이다.
1963년 보물 제1호로 지정되었다. 흥인지문은 성곽 8개의 문 가운데 동쪽에 있는 문으로 흔히 동대문이라고도 부른다. 흥인지문은 중간에 ‘之’ 자가 들어가 있어서 다른 문들과 달리 네 글자이다. 한양의 동쪽에 있는 낙산은 기운이 약하여 이를 보완하려고 이름에 청룡을 닮은 ‘之’ 자를 넣은 것이다.
흥인지문은 다른 성문들과는 달리 옹성으로 둘러싸여 있다. 낙산은 그리 높고 험한 산이 아니어서 다른 곳보다 적의 공격을 받았을 때 안전하지 못했기 때문에, 적에게 성문이 잘 보이지 않도록 반달 모양의 옹성을 쌓아서 지킨 것이다.

Hanyangdoseong Fortress was established to mark the downtown boundaries of Hanseong-bu, the capital of the Joseon Dynasty, and to protect the capital against invasions from outside. Heunginjimun Gate was built as the east gate of the Hanyangdoseoung Fortress in 1398 and was rebuilt in 1869. It was designated as Treasure No. 1 on January 21, 1963, and is commonly called Dongdaemun.
Attention needs to be paid to two aspects of the name, Heung_in-ji-mun. First, the letter “in” in Korean means to be broad-minded. Second, the name has four letters unlike other gates, as it contains the letter “之” in the middle. Naksan Mountain is located to the east of Hanyang, and its energy was weak to protect Hanyang. In order to supplement this weak energy, the letter “之“, which looks like a blue dragon, was added to the name of the eastern gate.
Another interesting point which distinguishes Heunginjimun Gate from other gates is that it was surrounded by a special structure called Ongseong. Naksan Mountain was not high and tough enough to protect the fortress and the gate from attacks by enemies, and a half-moon shaped Ongseong was built to make the gate seen not easily by enemies.