Great Gates of Azulon

The Great Gates of Azulon guard the sea entrance that leads to the Fire Nation Capital. Built early in Azulon's reign, the gates were named after the former Fire Lord, supposedly to remember Azulon's genius foresight in having them constructed (though in actuality for lack of a better option). An enormous statue of Azulon stands in the middle of the water, flanked on either side by two dragon statues. When enemy ships are spotted, a net rises between the statues and ignites -- thus "closing" the gate in a wall of fire.

During The Day of Black Sun, Part 1: The Invasion, the invasion forces approached the gates with the help of a fog cover-up made by Katara and the swamp Waterbenders. However, their arrival wasn't unnoticed, so the Fire Nation closed the gates before they could get past. When invading the oncoming invasion ships, the Fire Nation military was unable to find anyone on board due to the invasion forces using the ships as a cover-up for their submarines, effectively bypassing the gates.