Castle of Black Iron

The invisible sonic barrier in the air was like an extremely elastic film in front of Zhang Tie.

This film was not terrifying. Zhang Tie felt that he was completely able to break through it. However, Zhang Tie couldn't break through this sonic barrier even though he had repeated it many times.

The chakras of "King Roc Sutra" were still rotating stably and providing energy for Zhang Tie constantly. Zhang Tie felt that it was still not the highest speed that he could reach. He had not made full use of the flight ability granted by King Roc Sutra. What made Zhang Tie extremely depressed was that he could not break through 750 miles per hour no matter how he struggled although he felt that he could easily break through this sonic barrier. This speed was very terrifying for common knights; however, Zhang Tie was not satisfied with it.

It felt like that Zhang Tie could only drive a Faerie Dragon T9 on the highway at a speed of 40 miles per hour. However, the overall performance of Faerie Dragon T9 could allow it to run at a speed higher than 200 miles per hour. Additionally, Zhang Tie was also a racing driver. However, no matter how he struggled, he could only reach 40 miles per hour.

'What's the problem?'

If a sportscar suffered from this problem, the driver might not have selected a high-speed gear.

Zhang Tie also suffered from this problem when he could have a powerful flight ability granted by the King Roc Sutra. He sensed that he could easily break through the sonic barrier; however, he couldn't. There must be only one answer——There's a corresponding secret flight method in the complete King Roc Sutra which could help Zhang Tie unlock his powerful flight ability. As the King Roc Sutra that Zhang Tie grasped was incomplete, Zhang Tie had not grasped the corresponding secret flight method; therefore, he couldn't break through the sonic barrier.

Zhang Tie remembered that it was an intermediate layer above the stratosphere.

When Zhang Tie attempted to break through the sonic barrier, he felt two evident features in the intermediate layer: First, the higher it was, the colder it would be. In the middle part of the intermediate layer, it was 50 degree Celsius below zero. The strong wind was piercingly cold. Second, very evident convective motion.

The higher it was, the colder it would be. Being similar to that in the troposphere, the convective motion also existed in the intermediate layer, which was even stronger.

Zhang Tie didn't care about the low temperature at all because his protective battle qi and his cold-resistant ability were super great. As for Zhang Tie, the super strong air motion in the intermediate layer was completely like undercurrent under the sea, which could help him fly at a high speed with less strength.

Previously, he sensed air motions by the sharp instinct that he had cultivated physically and spiritually; now, with lotus-flower eyes, he could see clearly the directions of air motions in the intermediate layer.

In front of his lotus-flower eyes, the cyclones caused by invisible air flows in the intermediate layer were like super large color-changing mushrooms whose diameters varied from dozens of miles to hundreds of miles. It looked pretty splendid.

Zhang Tie had attempted many times. With the help of airflows, he could indeed fly more easily and more flexibly sometimes. However, no matter how strong the airflows were and how good he could use them, he could still not break through that sonic barrier.

When he reached above 270 miles high at the top of the intermediate layer, it was Zhang Tie's first time to see that faintly arc outline along the earth. When he caught sight of this arc outline, Zhang Tie felt a stress too. This stress was similar to the sonic barrier. Apparently, he felt that he could fly higher; however, he felt a transparent ceiling above his head, which prevented him from reaching higher.

Zhang Tie realized that he could at most reach the top region of the intermediate layer in the atmosphere above 270 miles high at the speed of about 750 miles per hour if not grasp the corresponding secret flight method of King Roc Sutra.

After flying 8 hours in the intermediate layer, Zhang Tie had long left the air territory of Longxi Prefecture, Zhongzhou Province.

After flying over 6,000 miles, Zhang Tie didn't feel tired at all. He only felt a bit depressed. He felt that the thunder hawk was darting towards him at a speed of about 2,000 miles per hour easily. 'As an earth knight who cultivates King Roc Sutra, my flight ability could not even match that of a bird?'

However, Zhang Tie knew that it didn't work only by being anxious. Unless he could gain a complete King Roc Sutra, he could only slowly think about the solution to make a breakthrough in his flight ability. However, as long as this complete emperor-level secret method appeared, it would cause great troubles between humans and demons as numerous powerhouses and forces would fight for it. Even heavenly knights and sages would be interested in it. An earth knight was even not qualified to attend such a competition.

After flying a few hours, Zhang Tie met 3 waves of knights and 2 airboats.

As the two airboats flew in the stratosphere, they didn't find Zhang Tie and just flew across the latter. As for the 3 teams of knights, one team contained 2 earth knights and 1 black iron knight. They also flew in the stratosphere. Although they discovered Zhang Tie above them, they didn't communicate with him and left directly. The second team contained 2 earth knights. When they caught sight of Zhang Tie, the two earth knights might be curious that Zhang Tie could fly in the intermediate layer of the atmosphere. They chased after Zhang Tie for a short while in the stratosphere and wanted to see Zhang Tie's face; however, Zhang Tie was not interested in communicating with them. He directly flew away.

The two teams of knights flew at a speed below 600 miles per hour in the stratosphere.

Additionally, Zhang Tie was shocked a bit by a shadow knight whose virtual image was a 9-headed weird snake in the intermediate layer of atmosphere...