What's Nurmengard?

         "Yes, I --- I did." She hesitated, looking upset, cradling her tea in her cold hands.
"I think that's the worst bit. I know Bathilda thought it was all just talk, but 'For the
Greater Good' became Grindelwald's slogan, his justification for all the atrocities he
committed later. And . . . from that . . . it looks like Dumbledore gave him the idea. They
say 'For the Greater Good' was even carved over the entrance to Nurmengard."
         "What's Nurmengard?"
         "The prison Grindelwald had built to hold his opponents. He ended up in there
himself, once Dumbledore had caught him. Anyway, it's --- it’s an awful thought that
Dumbledore's ideas helped Grindelwald rise to power. But on the other hand, even Rita
can't pretend that they knew each other for more than a few months one summer when
they were both really young, and ---"
         "I thought you'd say that," said Harry. He did not want to let his anger spill out at
her, but it was hard to keep his voice steady. "I thought you'd say 'They were young.'
They were the same age as we are now. And here we are, risking our lives to fight the
Dark Arts, and there he was, in a huddle with his new best friend, plotting their rise to
power over the Muggles."
         His temper would not remain in check much longer: He stood up and walked
around, trying to work some of it off.
         "I'm not trying to defend what Dumbledore wrote," said Hermione. "All that 'right
to rule' rubbish, it's 'Magic Is Might' all over again. But Harry, his mother had just died,
he was stuck alone in the house ---"
         "Alone? He wasn't alone! He had his brother and sister for company, his Squib
sister he was keeping locked up ---"
    "I don't believe it," said Hermione. She stood up too. "Whatever was wrong with that
girl, I don't think she was a Squib. The Dumbledore we knew would never, ever have
allowed---"