Trials

No Skyfiend from the Ten Tribes ancestral lands could ever pass the Wall. 

It is not because they could not fly higher than twenty feet. 

It is simply because there is an invisible barrier that causes the Dark Things that could fly unable to fly around the vicinity of the Wall

Those that crawl in the ground would feel pressured. Those who dig the ground could not dig the soil of the Wall.

There is no force in heaven and Earth and this is the meaning of that word. 

No creature that flies in the sky, no creatures that dig the Earth, or crawl the ground, none, should be able to bring down this wall.

Aegir shook his head and once again look at the gate and recognized something. 

Some people probably did not notice it. 

It is not because it is not obvious but most people would only look at the gate and the Wall for a brief moment before they quickly enter it