Ask the Maester: The Doom of Valyria, the Greatest Fighters in Westeros, and the Heartbreak of Greyscale «

What happened to Valyria?
Tormund mentioned that the remaining wildlings have gathered at Hardhome. Where and what is that?
I was wondering about the history of winters. How frequently do they arrive? How long do they last? More importantly, how much territory do they affect?