Fire Nation barricade

"water"

- You've got to hand it to the fire nation. They might be tyrants, but they certainly are fine engineers.

The Fire Nation Barricade was a large metallic barrier constructed by the fire nation in order to gain more control over the traffic through the gap in the Serpent’s Pass. The barricade separates between the East and West lakes and therefore limiting the traffic.

History
After the Earth King was overthrown, the military of the Fire Nation, under the command of Admiral Liang, quickly swarmed into East Lake, forcing the Water Tribe forces, along with Aang and his friends, to abandon their defense of a nearby bay. After scuttling the Water Tribe fleet in the center of East Lake and commandeering a Fire Nation patrol vessel, they fled west, and later encountered the new barrier built by the Fire Nation. Hakoda managed to trick the sentry atop the barricade into thinking that they were rendezvousing with a fleet at the other side, and thus succeeded in passing the barrier.