My question is Why does Robin warn Adam to keep his plan secret to save the castle?

From the text:

"Let me go," said Robin. "I can go out the small door at the north whilst it is early morning. No one will suspect me. They think me a poor shepherd. I shall borrow a smock from William the Farrier's son, and if I am seen, I shall appear stupid. We shall keep it secret, for if Sir Peter were to find out my plan he would forbid me to go, not knowing how strong I am."