Which castle is the most difficult?

For beginners, Necropolis. To play well, you need to be able to produce skeletons. And for that, you need to be able to conduct battles of varying difficulty, with minimal losses, and strategically position chains of heroes, etc. I would put Inferno in second place; playing with demons requires a certain skill. The units are weak, and it's very difficult to manage without significant losses at the beginning. It's easier for beginners to manage cities that have a good ranged army at the start. Managing melee units is a bit more complex.