Eret's Fort was once a sturdy outpost for shipping dragons, complete with docks for ships, cages to house captive dragons and weapons. There are even raised docks for archers to fire and capture dragons that are flying by.
But since the attack, the fort was been reduced to splinters and ice spikes. Eret and his fellow trappers since reside there in the beginning of the second film, are ready to attack any dragon or rider that passes by, to make up for the dragons they've lost.
Eret's Fort was once a thriving outpost for trappers to trade and ship dragons to Drago. But Valka's Bewilderbeast destroyed the fort (as Valka and Cloudjumper saved it's captured dragons), leaving it in a frozen wreckage by the ice spikes and splinters; serving as a warning to dragon trappers everywhere.
Hiccup and Astrid noticed smoke from Itchy Armpit and went to investigate. They had unknowingly fell into the traps of the dragon trappers and Stormfly was caught. Eret makes his first appearance here and accused the duo of wrecking the fort. Luckily, Toothless blasted the ice spikes, making time for Hiccup and Astrid to free Stormfly.