From William Shakespeare’s Henry VI, part 1 (I: 4):
The prince’s espials have informed me
How the English, in the suburbs close intrench’d,
Wont, through a secret grate of iron bars
In yonder tower, to overpeer the city,
And thence discover how with most advantage
They may vex us with shot, or with assault.
espials: spies; the act of noting, observing, or taking into account