The Secret Dock was a dock found at the back of Kingshead, not much special about it except that it was very secret, it was accessible by swimming around the island or doing a specific glitch to reach it but with the July 2010 updates to Kingshead, the dock was deleted.
Secret Dock History
Pirates remember seeing a cave at the Secret Dock somewhere around 2009. When there was, the pirate lands in the ocean on the other side of Kingshead after the glitch. The dock is nearby, and a cave was at the entrance, The pirate could just swim toward it. When the pirate exits through the entrance, he/she is on the dock. A year later, the cave went missing. Some say this is because most pirates perform another glitch. It erased all of the enemies there so the pirate could easily get to the Secret Dock without being ambushed, and if they do this, they could not see the cave. Others say the Pirates Online Team erased the cave, so they couldn't see the cave either way. As of July 2010 it is on speculated on why the Disney Team deleted the Secret Dock.
The existence of formerly the dock and the cave, lead to ideas of what they were for. Understandably, they were likely unfinished. Either as a back entrance to the fort or possibly part of a future quest.
Deep inside the fort, there is a blocked-off hallway which goes nowhere, BUT comes up to where the glitched cave entrance ends. Likely the two were meant to be joined.
So, at some point - pirates were likely going to either enter or exit the fort through this back door.
This may also tie into why Kingshead has an unused jail. Perhaps a future quest will have the pirate either breaking out of or into this secret jail and escaping via the cave entrance to the secret dock.
It is also rumored that the Cave is still inside the fort, but blocked off. BUT this has not been proven.