I have actually wondered what they were there for myself. The last time I was in one the wiki said it was WIP, so I figured more would be done later.
It's in the mod now. You can access it but I've heard the boss is buggy. The armor added looks cool.Yeah the Twilight Forest always has new stuff every time I look at it
http://twilightforest.wikispaces.com/Goblin Knight Stronghold I wonder if this is incorporated yet? It sounds like it will be awesome!