故宫内的殿宇楼阁均为砖木结构，很容易着火。一旦发生火灾，如果不能及时扑救，火势就会迅速蔓延，把这座价值连城的辉煌建筑顷刻之间化为乌有。为此，紫禁城的建设者们对防火特别重视，在宫殿前设置了这些铜大缸，称之为“门海”。门海即门前的“大海”，门前有了“大海”，以水克火，砖木结构的故宫就不怕火灾了。因此，这些铜缸又被称作吉祥缸。每尊缸可贮水3000多升，如同一座座小水库。
The palaces and pavilions in the Palace Museum are all brick and wood structures, which are easy to catch fire. Once a fire breaks out, if it can not be put out in time, the fire will spread rapidly, and this magnificent building of great value will be vanished in an instant. For this reason, the builders of the Forbidden City attach great importance to fire prevention, and set up these large cylinders in front of the palace, which are called "Menhai". Menhai is the "sea" in front of the door. With "sea" in front of the door, the brick and wood structure of the Forbidden City will not be afraid of fire. Therefore, these copper cylinders are also called auspicious cylinders. Each cylinder can store more than 3000 liters of water, just like a small reservoir.