LONDON (AP) — Dorothea Barron got a preview of the D-Day invasion from a watchtower on the coast of Scotland.

During the spring and summer of 1943, she and her colleagues kept watch over the troops as they tested the prototypes for two portable harbors that would be used at Normandy to ease the delivery of men and equipment to the battlefield. Whenever someone got into trouble, Barron would unfurl her semaphore flags and signal for help.

“We were watching over them, shall we say,” she told The Associated Press. “If they got into difficulty, we would then inform their headquarters, their people on the land. ‘Boat so and so has been hit’ and things like that.”

As an 18-year-old rating in the Women’s Royal Naval Service, Barron was a small part of the team that developed the Mulberry harbors, the system of breakwaters, pontoons and floating roadways that were built on the coast of Normandy. Some 2 million soldiers, 4 million tons of supplies and 500,000 vehicles rolled through the two harbors by the end of 1944, contributing to Allied victory in the Battle of Normandy.