Read the given passage and answer the questions that follow.
The beaver had made a dam about hundred yards long, and it had formed out of the quickly flowing stream, a wide stretch of water about twelve feet deep. The dam was so firm and broad that it was easy for me to walk along it. After a hundred and fifty yards I came to a 'beaver's castle'- a great heap of logs skilfully fitted together, the lower part covered with earth and plants. The upper logs were put loosely together so that the air could pass through to the interior.
In the late autumn, before land and water are covered with snow and ice, the beaver has to make its dwelling frostproof. It collects young trees and gnaws the stems in a double cone until the tree falls. Then the long stems are gnawed into small sections, the thin twigs bitten off and the logs are dragged to the dwelling. There, the timber is piled on the castle, sometimes to a height of ten or twelve feet. Mud is brought up from the bottom of the lake and packed between the logs with the animal's clever forepaws. The interior is lined with the finest wood shavings. In this tall shelter, the beaver remains high and dry, and protected from frost throughout the winter.
The only access to the dwelling is under the water, and even the forest wolves are not able to pull the firm building apart. The water is the beaver's element. On land it moves slowly and awkwardly. Thus, it is vital for the beaver to have water in which it can swim, and where nature has not provided this condition for it, it creates it with its dam.
The beaver's activities can change whole landscapes. With this dam, for instance, on which I was standing, beavers had turned a whole wooded alley into a lake; trees that stood there had been killed by the water and had disappeared. Aquatic game had settled there. Ducks swam past us, and great must have been the number of fish, as swarms of trout had swum past me in the clear stream. Wide stretches of meadow had come into existence on the banks, with flat landing places to which the well-beaten beaver tracks led. Many years of building, gnawing and dragging must have gone into the completion of this immense task, creating a new region for the beaver to live in.
How does a beaver ensure that its castle is well ventilated?