Fred had to admire the excellence of the construction work.

The descent, as he knew from what he had seen outside, must really be very sharp.

But it was managed here with turns and zigzags so that the grade was never very sharp.
Fred became suddenly conscious of a change in the air.
"I must be near the opening," he thought.
A couple of minutes proved that he was right.

He now remembered that Boris had not had time to tell him how the door or gate was operated.
But he decided not to go back at once, but to try to discover the secret for himself.

It had occurred to him that it was more than probable that a sentry or two might be left in the house, and he had no mind to stay in the passageway, helpless and useless, if Vladimir found it impossible to let him out at once.
At the end of the passage he found a solid, seamless door.