Here were two distinct groups of giants called the Cyclopes, whose name means circle-eyed and indicates their principle distinguishing feature, one round eye in the center of their forehead. The first, offspring of Uranus and Ge, were the smiths who labored with Hephaestus at his forge to create the thunderbolt for Zeus, among other masterpieces. The second group of Cyclopes were a tribe of giants, the most important of whom is Polyphemus, a son of Poseidon, encountered by Odysseus. The word cyclopean refers to anything that pertains to the Cyclopes or partakes of their gigantic and powerful nature. Thus the Cyclopes were said to be responsible for the massive stone walls which surround the palace-fortresses of the Mycenaean period. And so cyclopean is used generally to describe a primitive building style, which uses immense, irregular, stone blocks, held together by their sheer weight without mortar.