In the time of ancient Thassilon, Thistletop was the head of an enormous giant statue, likely to be nearly two hundred feet tall. The head, and maybe the entire statue had room carved into it.
Then, some time in the past, a catastrophe overtook the Thassilon emprire, and it is likely that the statue was destroyed at the same time. Only the head now remains, poking about 60ft out of the sea. The head has a fairly flat top, and the Goblins built a small fort on the top. They also took over the rooms within the head. The Goblin had connected the head to the mainland with a rope bridge.
There are a number of ways to get into the head, with at least three known to the party. Two underwater, and one in the top of the head.