He takes the initiative because he needs to show the rest of the tribe that he is still in control, he also needs to show the kids that because he is the leader he has to do things that other are afraid or that they don't want to do
Sam and Eric initially follow Jack's orders and inform him of Ralph's arrival. However, they struggle with their loyalty to Ralph and end up warning him about Jack's intentions to hunt him. They ultimately help Ralph escape from Castle Rock.
Sam and Eric warn Ralph that Jack and the others are going to hunt him tomorrow. There for they are going to kill him
Ralph fights with Jack. (epic)
Ralph sees the figure of the naval officer on top of the castle rock. He mistakes him for the beast before realizing his true identity.
Ralph learns of Jack's plans for him through Samneric, who are forced to join Jack's tribe and spy on Ralph's group. Samneric bring Ralph news of Jack's intentions to hunt him down and kill him. This betrayal shows Ralph the extent of Jack's animosity towards him.
Ralph goes first when the boys reach castle rock because Ralph is the elected leader and he feels that it is his duty.
Roger does not say anything to Ralph as they approach the "rock castle" in the Lord of the Flies. Instead, Roger silently throws rocks at them from above, demonstrating his cruelty and violent tendencies.
Samneric tell him while they are on guard at Castle Rock.
Ralph says it's a rotten place because there's no running water there.
Jack is feeling guilty just as Ralph and Piggy are.
When Ralph recognised from their silhouettes that Samneric were on guard duty on top of castle rock he took the risk of climbing it to speak to them. Ralph knew that Samneric had remained loyal right up to the point when they had been catured and forced to comply with Jack's wishes.
It´s not a a sand castle it is a rock castle and it is Jack the leader who later makes everyone else follow him. I you are talking about he shelters it was Ralph.