The furnace (溶鉱炉,Yōkōro?) is an area underneath the castle.
There is a lever which opens the furnace, but will close after a few minutes. Here that Leon S. Kennedy fought two Gigante, before proceed to the Pit.
Leon must draw the two gigante and climbed into the floor. Once the Gigante reaches close enough, slide across the area. There is a lever which open the furnace, pull it once the gigante step on top of it. Once done, one gigante will fall and died from the lava, leaving only one gigante to fight. Remember to keep distances from the furnace, because the gigante can make one final attack before dying.