A flower girl

Why does a tower that kills gods exist in the realm of heaven? Does it serve any purpose? Or is it just a plot of the god of dungeons to prepare the gods for his ultimate plan?

No, before the tower was created, he created dungeons so everyone can hunt, test his strength and aim for the upper realms. His main objective was to make heaven a peaceful place where all gods lived the same. He didn't want to see any discrimination between gods and immortals so he set up dungeons. 

But everything comes with a price. 

Soon, so many gods and demigods died and the king gods announced that everyone should stop going inside. But the dungeons remained as they were.

After a while, a huge tower appeared in the middle of heaven that pierced through every sky realm. The civilians of the heaven were curious but when they found out it was a tower made by the god of dungeons, they quickly left the place and the tower remained untouched.