Gheor wasn't an idiot, nor was he underestimating the Imperial Army. He knew full well that the Empire had prepared several plans to take down the Tiger Corps.

In spite of that, he had chosen to fight for two reasons.

First, it was his responsibility to protect the cities and their citizens. 

Gheor had dispatched the Eagle Corps and the Elephant Corps in a way that'd protect the free cities. However, that had been exactly what the Empire had wanted.

Second, he knew that the Empire sought to destroy the Tiger Corps, so he struck at their weakness.

They would use 'airships' to accomplish this! These airships were designed by Gheor and Han Seol, and the Storm Guild had created them.

It was finally time to use them. Instead of sails, they flew using giant balloons.

Plus...

Kyaaaahk!

Each ship was pulled by ten wyverns. Their job was to drag the airship in the right direction, as well as guard them if necessary.

It was a method of transportation that hadn't existed within Arth! Moreover, they were traveling through the sky!

That's why they were able to transport the Eagle Corps and the Elephant Corps without being detected.

Of course, if you took a good look at them, you'd notice how crude they were. They used an actual boat as the hull of the ship, and it was pretty clear how much of a rushed job the balloons were.

Still, there was no better...