The main tower of the Sovinec castle

Overheard at another historic site:
  "Mommy and what is this castle?"
"A castle is a castle!" replied the young mother, peering into the depths of her cell phone screen.
  "And why does the castle have such a big round tower?" the curious child was restless.
  "I don't know… and stop bugging me or I won't buy you any ice cream!" snapped the parent, continuing to surf Facebook on her smartphone.
It was…