Definition of "watchful"

But there was so much at stake that I had to take some risk—though even when I was far away there has never been a day when the Shire has not been guarded by watchful eyes.

1954 July 29, J[ohn] R[onald] R[euel] Tolkien, “The Shadow of the Past”, in The Fellowship of the Ring: Being the First Part of The Lord of the Rings, New York, N.Y.: Ballantine Books, published September 1973, page 93