The Labyrinth is the main fighting place in Summon Night: Swordcraft Story. It is filled with Stray Summons and is the only place in Wystern where one can successfully gather forging materials. It is made of 100 floors and reaches the bottom of the sea.
The Labyrinth was constructed along Wystern a long time ago, when humans used Summon Beasts only as tools to make stronger weapons. It is where Parista, the Holy Spirit of Fire was sealed at that time. Periodically, Craftlords would descend through the Labyrinth in order to keep Parista sealed. Also, the Labyrinth used to connect the lower levels of Wystern before they sank, thus being the Central Tower in ages past.
Kuhyra is located in the right-most room in the 50th floor. Talking to him will allow you to switch your current Guardian Beast with Kuhyra.