I haven't even reached this mission yet and decided to read the walkthrough; after reading what you all are writing here, I actually got scared, and at the same time, I still didn't understand how to complete it correctly...

Added after 4 hours 22 minutes
I went straight to Hork, smashed the siege, and took the city. Then I fended off a demon attack; meanwhile, I cleared 1 garrison with dwarves, and using Frida, I gathered armies from everyone, resulting in a human army with 6 dragons and 21 witches, then I cleared the 2nd garrison with dwarves—the same as before, no one was left in the dwarf army. I entered the Castle with Frida, but there I was met by such "cheerful" guys that it made me feel sick; during the siege, I demolished all fortifications and only wiped out all the Jarls and dragons from the defense, others were just heavily scratched. What should I do? I still have about a month in reserve, but the defense will keep growing...