This is how the trouble began. From the beginning of time, the giants had been unfriendly to the Æsir, because the giants were older and huger and more wicked; besides, they were jealous because the good Æsir were fast gaining more wisdom and power than the giants had ever known. It was the Æsir who set the fair brother and sister, Sun and Moon, in the sky to give light to men; and it was they also who made the jeweled stars out of sparks from the place of fire. The giants hated the Æsir, and tried all in their power to injure them and the men of the earth below, whom the Æsir loved and cared for. The gods had already built a wall around Midgard, the world of men, to keep the giants out; built it of the bushy eyebrows of Ymir, the oldest and hugest of giants. Between Asgard and the giants flowed Ifing, the great river on which ice never formed, and which the gods crossed on the rainbow bridge. But this was not protection enough. Their beautiful new city needed a fortress. So the word went forth in Asgard,--"We must build us a fortress against the giants; the hugest, strongest, finest fortress that ever was built." Now one day, soon after they had announced this decision, there came a mighty man stalking up the rainbow bridge that led to Asgard city. "Who goes there!" cried Heimdal the watchman, whose eyes were so keen that he could see for a hundred miles around, and whose ears were so sharp that he could hear the grass growing in the meadow and the wool on the backs of the sheep. "Who goes there! No one can enter Asgard if I say no." "I am a builder," said the stranger, who was a huge fellow with sleeves rolled up to show the iron muscles of his arms. "I am a builder of strong towers, and I have heard that the folk of Asgard need one to help them raise a fair fortress in their city." Heimdal looked at the stranger narrowly, for there was that about him which his sharp eyes did not like. But he made no answer, only blew on his golden horn, which was so loud that it sounded through all the world. At this signal all the Æsir came running to the rainbow bridge, from wherever they happened to be, to find out who was coming to Asgard. For it was Heimdal's duty ever to warn them of the approach of the unknown. "This fellow says he is a builder," quoth Heimdal. "And he would fain build us a fortress in the city." "Ay, that I would," nodded the stranger. "Look at my iron arm; look at my broad back; look at my shoulders. Am I not the workman you need?" "Truly, he is a mighty figure," vowed Odin, looking at him approvingly. "How long will it take you alone to build our fortress? We can allow but one stranger at a time within our city, for safety's sake." "In three half-years," replied the stranger, "I will undertake to build for you a castle so strong that not even the giants, should they swarm hither over Midgard,--not even they could enter without your leave."