Henry of Lancaster, Earl of Derby, may be called one of the earliest great English military engineers, for in 1345, the year before Crecy, he first taught the troops under his command how to set about fortified towns during the French campaigns, using saps and tunnels and mines to approach and blow up walls.
At this period young boys were often employed by these army engineers as “Listeners”. This was most dangerous and unpleasant task, for it meant that when a tunnel had been dug under or near to the enemy’s position, one of these lads was posted at the end of the sap to listen for knocking or other sounds which might show that miners of the opposite side were also working underground and likely to take the besieging force by surprise.

Besides the likelihood of being discovered or blown up by enemy mines, there was always the chance that a tunnel-roof would collapse upon the unfortunate listener. This happened one night when the English were besieging a town in France, but the boy who had been posted to listen to the end of the sap managed to scramble clear of the ruins and found himself in the cellar of a house inside the besieging town. Stealing out into the street, he discovered a postern gate which could be opened from within and through this he admitted the besiegers.