To complete this area you must defeat the Ice Worm twice and collect all four power crystals to light the furnaces. On your adventure, Bok will teach you new spells and give you new spells and items on your adventure. Also, you have to fight the Ice Worm at least once before you can light up all the furnaces.
Bosses - Ice Worm
When entering Bok's Cave, you have three paths, down, right, or left. Down will bring you to Borderland, where a Cloaked Wizard may attack you. Right will bring you to the Furnace Room. There, power up the four furnaces using Power Crystals. When you complete all of the quests, go up into a new room, the Hidden Room. Going left from Bok's cave will bring you through various areas.
You go into the Treasure Room when on the left path. There's even more split paths...go to the Rail Ruins or the Mountain Pass. The Rail Ruins brings you to the Mountain Valley, the Worm's Rest, the Frozen Pass (go left into The Chasm or right into the Royal Passage, splitting into either the Throne Room or the Shiverchill Gate). Going through the Mountain Pass will bring you through the Ruined Entrance, Ruined Passage, the Ruined Steps, and loops back to the Rail Ruins, oddly.