When you reach the bottom-left corner of the map, you will encounter the boss of the section just after it crushes the Giant Bat.
Balore will attack with his two fists, by either swinging them towards the center, or by smashing them against the ground. While the telegraph is obvious in both cases, they can be dangerous if they do not approach at the same time. If desired, you can even ride the fists to attack Balore's eye.
When Balore is low on health, he will switch to his other eye. This glowing eye will shoot a beam against the ground, causing a fire to follow. It is dodged by jumping over the flame.
After Balore is killed, you can collect the Giant Bat soul. When you return, you will speak with "J" to learn about his real name and his intent - he will retrieve the vampire slayer, and state his mission to destroy Dracula.
With the Giant Bat soul, you are free to fly anywhere, collecting the items that were previously too high (or were in a small location), and reach the Top Floor. While there are a few locations in the castle that are yet to be explored, they are optional (especially if you know what is needed to complete the game.)