The Crooked Forest (Gryfino, Poland) was visited by Black Swan members Sophie, Calla, Fitz, Biana, Keefe, and Dex in the fourth book. It was the meeting place for them to go after Exillium, where Calla escorted them back to Alluveterre. Calla says she sang to the trees, which was why the trees were bent. It is a Forbidden City.
Judging from the name, it's probably a forest. The trees there are crooked and bent.