The Fortress Island

The Fortress island is a part of the Forgotten Isles.
The Fortress island is said to be protecting the Never-ending Sea from anyone who seeks its treasure.

Not much else is known about the fortress. One thing to note is the strangely blue stone that the fortress is built on. There could be a possibility that the Fortress and Never-ending Sea could be expanded on in one of Sid's games or a sequel.