Once Yerba Buena and Treasure Islands fell under the control of the United States Navy, they quickly established a powerful military base that sprawled across both islands. A key component of this base was a strategically-placed observation tower that remains atop Yerba Buena Island to this day.
The tower, which arguably offers the best views of the San Francisco Bay, was originally used by the Navy in the 1940s as a control tower for seaplanes. Decades later, the tower was converted into an “officers club” (a bar restricted to military personnel only) known as The Tower Club. Occupancy was controlled by a stoplight at the base of the entry ladder: a red light meant the Tower was closed, green meant it was open, and yellow meant that it was open but full and to come up only after a patron came down. Unfortunately, the bar was closed and the tower condemned following the 1989 earthquake.
Update: As of 7/24/2016, all that remains of the tower is its foundation.