It was an aerodrome, and a secret one.

The place had been most cunningly chosen.

For suppose anyone were watching an aeroplane descending here, he would think it had gone over the hill beyond the trees.

As the place was on the top of a rise in the midst of a big amphitheatre, any observer from any direction would conclude it had passed out of view behind the hill.
Only a man very close at hand would realize that the aeroplane had not gone over but had descended in the midst of the wood.

An observer with a telescope on one of the higher hills might have discovered the truth, but only herds went there, and herds do not carry spy-glasses.