Red Plain
There were many reasons why Ryu had not chosen to take on the challenges of the city gates himself and instead chose to circumvent them. Many of these reasons were kept to himself, but of them, maybe the most important was his stamina.
After watching Sarriel's battle especially, Ryu could tell that the purpose of the gate trials was less about testing for talent, and more about testing one's limits. If not for this and being pushed right to the bitter end, how could it be that Sarriel would have been so tapped out?
With her talent, and especially her prowess, this was impossible.
To Ryu, these trials were nothing more than pokes and prods, as though someone from on high was watching them for entertainment. Toward such things, Ryu had no patience. If he was going to put himself on the line, it would be for something meaningful. And, this Keep was exactly one of them.