Walkthroughs for custom maps.

After the Necropolis appeared, I went to the purple castle. There's a garrison with elementals below, but to open it, I need to open the green guard first. However, even after that, a Necromancer attacks through this garrison, and I can't defeat him at the moment. There's also an AI with black dragons and Cerberuses + a Sphere of Inhibition. What should I do? Also, I don't have 18 dragons to open one of the guards.