I am trying to find a book I read in elementary school and I am trying to figure out the title and/ or author.?

The book is about a children who went to some sort of island. There is a king, like in most Juvenile stories. The book talks about odd places and people and about numbers and letters. I distinctly remember there being a tower in the story that they had to climb. It is not by C.S. Lewis or Roald Dhal ( pardon if I didn't spell that correctly ) but seems like something he would write.