This is what remains of the old fort in Lahaina. It was built in 1827 to protect the town from angry sailors. Apparently, the Rev. William Richards who was a missionary in Lahaina, converted the natives to Christianity and then told all the ladies they should no longer go out to the whaling ships and, ahem, entertain the sailors. The ladies were coming back from the ships with pretty beads and nasty diseases. One of the ship's captains fired a cannon at Rev. Richard's house in retaliation. No one was hurt but the royals at the time felt that a fort was needed to help protect the townspeople from any further incidents.
The fort covered nearly an acre and was constructed of coral bricks. Visitors to Lahaina remarked that the fort seemed more for show than for protection and that it was primarily used for firing gun salutes on the king's birthday. It was torn down in the 1850's and the blocks were used to build the town's first prison.
Better to lock them up than to just block them out!