how i reach to the secret level in book 5?...

[vlad_k]

the battle arena where i can choose all those heroes :O

[muaddib]

i think you need to do at least 2 things for that one:
the first rescue all surter's people [40 in total] and reach to safety on time.
second condition is to defeat afterwards , ofc, clech the cog master, once he dies a red question mark should pop.
you have to pick it fast, however.

[Hatori-san]

Also you have to find Prince Galar in Book 4, the chapter with Ziel, Kanna and Shizu. He's standing on top of a mountain, only reachable with Ziel's whirlwind-form.