The Broken Road by A.E.W. Mason

It was the mine underneath the North Tower which brought the career of Luffe to an end. The garrison, indeed, had lived in fear of this peril ever since the siege began. But inasmuch as no attempt to mine had been made during the first month, the fear had grown dim. It was revived during the fifth week. The officers were at mess at nine o'clock in the evening, when a havildar of Sikhs burst into the courtyard with the news that the sound of a pick could be heard from the chamber of the tower.

"At last!" cried Dewes, springing to his feet. The six men hurried to the tower. A long loophole had been fashioned in the thick wall on a downward slant, so that a marksman might command anyone who crept forward to fire the fort. Against this loophole Luffe leaned his ear.