Goblin Knight Stronghold
These structures are found underground in the Dark Forest. There is a shield blocking anyone from entering, but there is a pedestal which opens a small hole if you put a trophy on it.
At first, it is a small passageway, but as soon as you get to the entrance, there is a huge maze consisting of many rooms, like feast halls, small passageways, atriums, foundries, and more. They do have good loot, like Knight Metal and Knightly armor, but also are home to deadly monsters that are out to get you. They are vaguely similar to Labyrinths in some ways.
The monsters that inhabit the stronghold include Creepers, Kobolds, Redcap Goblins, Redcap Sappers, Slime Beetles, Helmet Crabs, Block-and-Chain Goblins, and Goblin Knights.Once you get to a room covered in iron bars, you will meet the Knight Phantoms boss. The boss fight consists of 6 ghostly floating pieces of Phantom armor that float about and smash into you, as well as throw Knightly tools at you. Once you kill all 6, a treasure chest with valuable loot will spawn.
Progression
Using the progression system, the area beneath the trees in the Dark Forest will be filled with smoky ribbons of dark magic. You will be affected with blindness if you stay in the area. Defeating the Hydra will allow you to enter unimpeded. You will also need the Hydra Trophy to unlock the lower levels of the Goblin Knight Stronghold.
Defeating the Knight Phantoms will allow you to interact with and enter the Dark Tower at the center of the dark forest.