The Eyes of the Dragon by Stephen King

In the kingdon of Delaine, the old king Roland the Good, finds himself near the end of his life, without an heir to the throne. He marries a young woman named Sasha, who bears him his first child named Peter. There is magician who lived for hundreds of years, always leaving Delain and returning under a new name, to plunge Delaine into chaos. This time around he comes as Flagg the kings advisor. Flagg is content with Roland as king, for alhough Roland is good man he is an idiot. An idiot who can be controlled by a man as cunning as Flagg. However when Peter is born, Flagg instantly realises the threat to his power. Peter is brave, and intelligent, the stuff of legends. Peter would not be controlled as Roland was.
The cunning magician secretly plots to kill Roland, and frame Peter. Then Peter's brother Thomas five years younger than he, would become king. Thomas was not like his brother he was bumb and was terrified of Flagg. Flagg could use this to control him. Peter imprisoned in the Needle, a 300 foot tower, meanwhile plots a daring escape, with the help of his friend Ben and without him knowing it, the same man who had convicted him guilty of murder, Judge General Anders Penya.