All resources and buildings are needed to reach the maximum castle level. The castle is central; you must level it up to unlock other buildings and better armies. So there should be no other targets until you reach max castle level 30. This will require a lot of resources, so focus on resource production.
Clash of Kings Castle Upgrade Requirements
Castle Prestige 1 Requirement
Requirements: Level 30 Castle, Level 30 Wall, Level 30 Farm, and Level 30 Wishing Well
Castle x10 Upgrade: Wonders, 4.43M Lumber, 4.43M Food, 422k Iron, 27k Mithril, 100 Prestige Badges
Castle Prestige 2 Requirements
Requirements: Level 1 Advanced Wall, Level 1 Advanced Iron Mine, Level 1 Advanced Lumber Mill, Level 1 Advanced Mithril Mine
Upgrade Castle x10: Wonder Prestige Level 1, 7.3M Wood, 7.3M Food, 695k Iron, 44k Mithril
Castle Prestige 3 Requirements
Requirements: Level 2 Prestige Wall, Level 2 Prestige Depot, Level 2 Prestige Farm, 1.1mil Iron, 9 Prestige Badges
Upgrade Castle x10: Wonder Prestige Level 2, 13.3M Lumber, 13.3M Food, 1,25M Iron, 400 Prestige Badges
Castle Prestige 4 Requirements
Requirements: Level 3 Advanced Hospital, Level 3 Advanced Military Tent, Level 3 Advanced Sawmill, 1.96m Lumber, 494k Mithril, 18 Advanced Badges
Castle x10 Upgrade: 20.1M Wood, 20.1M Food, 1.91M Iron, 120k Mithril 760 Prestige Badges
Castle Prestige 5 Requirements
Requirements: Level 4 Advanced Stables, Level 4 Advanced Barracks, Level 4 Advanced Iron Mine, 3mil Iron, 750k Mithril, 32 Advanced Badges
Upgrade Castle x10: Wonder Prestige Level 4, 24.7M Lumber, 24.7M Food, 2,35M Iron, 148k Mithril 1100 Prestige Badge
