"It uses our emotions as fuel." ... wait, that's probably not going to make much sense to a rabbit. "Er-- I mean.. it's kind of like the same thing as us needing to eat to stay alive. But the castle doesn't really eat us, it just... gets its energy from our emotions somehow."

A small smile at the rabbit. "To tell the truth, not even the humans here really understand how the castle works." ...Tsuna, that's probably not comforting.