The focused energy of light.

I had to go back and mention another gift from my old buddy Archimedes. This Greek from Syracuse is said to have developed a series of large mirrors he could position on shore overlooking an approach to the City's harbor. He was able to focus the light of the sun with these mirrors to either blind ships' tillermen or start the ships on fire. The first Death Ray(!) three centuries before Jesus.

A simple, elegant defensive solution almost no troops required to operate it. Risk of the loss of life in minimal on the part of the defenders. Low cost. I'm always awestruck by the genius of Archimedes. Sad to say a Roman soldier killed Archimedes as they overran the city despite specific orders to spare Archimedes because he was so brilliant! <|:-{(#