The lamp gallery was at the top of Fang Rock lighthouse. There, personnel could lookout and view the light. It had a direct line to the generator room below. Vince Hawkins saw a Rutan scout ship hit the water while looking out from the gallery.
Reuben and the Fourth Doctor watched from the vantage point as Henry Palmerdale's steam yacht crashed against Fang Rock. Leela saw the Rutan while looking into the water. She asked the Doctor if it was the mythical Beast of Fang Rock. Palmerdale was killed in the lamp gallery after the Rutan scout scaled the wall and electrocuted him.
Acting on Leela's suggestion, the Doctor turned the light in the lamp gallery into a converter to create a carbon oscillator. Leela used a makeshift mortar, which was set up in the gallery, to kill the scout. The oscillator destroyed the Rutan mothership. (TV: Horror of Fang Rock)