Fortress.
Frozen Island was about 100km away from the City of Coldland.
It was inhabited by dangerous Mutated Beasts, and each one of them can cause a lot of havoc among civilians.
Fury Beasts are about Martial Captain and Peak Martial Commander level in strength— that alone is terrifying existence, and there are hundreds of them on that island!
But gladly, there is Martial General guarding the island.
And if some of the Beasts escape. People from Coldland will get that news very quickly, and since it's 100km away, the Beasts will take some time before they arrive there, and Coldland would have already gathered their fighters to exterminate the Beasts.
The Frozen Island itself is quite large, around 100km in diameter, filled with snow forests, snowy mountains, and cold weather.
It is a place where ordinary fellows can't go, and no ordinary fellow even wants to go there!
But it's the perfect place for training for Martial Arts Practitioners.