Also, as they had come up the aisle of the woods she had been assailed by a late but serious doubt, whether a weight-carrying motor-car was quite the right kind of vehicle in which to approach the lair of the Twins with hostile intent.

Its powerful, loud-throbbing engine had seemed to her to advertise their advent with all the competence of a trumpet.
Her doubt was well-grounded.

The quick ears of Erebus were the first to catch its throbbing note, and that while it was still two hundred yards from the entrance of the path to the knoll.

Ever since the departure of Miss Lambart and Sir Maurice the Twins had been making ready against invasion, conveying their provisions and belongings to the secret caves.
The secret caves had not been secret before the coming of the Twins to the knoll.

They were high up on the outer face of it, airy and well lighted by two inaccessible holes under an overhanging ledge.