Which castle is the most difficult?

In my opinion, there's nothing complicated about mastering any castle! You start a game in a random map against the AI, enter a castle, and click on everything with the right mouse button, while carefully reading and studying! This is necessary to figure out on what day you will build what (this is important, I think). There is no standard development tactic, and there won't be. Regarding which castle is expensive and which is not too demanding, it all depends on the wealth of the starting zone. You also need to estimate based on the resources on what day you want to build what and how many of which resources you will need. Accordingly, collect/defend them. Play more often and in any conditions, save/load, and make it more challenging!