But Karl had foreseen this difficulty, and provided against it before a stick of timber had been cut.

Indeed, close following on the first conception of the scaling ladders, this matter had passed through his mind, and had been settled to his satisfaction.

Only theoretically, it is true; but his theory was afterwards reduced to practice; and, unlike many other theories, the practice proved in correspondence with it.
Karl's theory was to make the holes by fire--in other words, to bore them with a red-hot iron.
Where was this iron to be obtained?
That appeared to offer a difficulty, as great as the absence of an auger or a mortise-chisel.
But by Karl's ingenuity it was also got over.

He chanced to have a small pocket pistol: it was single-barrelled, the barrel being about six inches in length, without any thimbles, beading, or ramrod attached to it.

What Karl intended to do, then, was to heat this barrel red-hot, and make a boring-iron of it.