midnight
On several occasions Tarzan had thought that he discerned things moving behind the ruined portions of the wall near to them , as though creatures were watching them from behind the bulwarks of the ancient pile . And often he felt the sensation of unseen eyes upon him , but not once could he be sure that it was more than imagination . That night they camped outside the city . Once , at midnight , they were awakened by a shrill scream from beyond the great wall . It was very high at first , descending gradually until it ended in a series of dismal moans . It had a strange effect upon the blacks , almost paralyzing them with terror while it lasted , and it was an hour before the camp settled down to sleep once more . In the morning the effects of it were still visible in the fearful , sidelong glances that the Waziri continually cast at the massive and forbidding structure which loomed above them .