What are you talking about? We don’t have a Citadel. The only structure that reaches level 25 is the Stronghold. And even level 25 Stronghold does not produce anything, no hero, troop, food or iron. It may give you gems if you are on VIP. However, a level 25 Stronghold allows player to unlock the Hero Academy.
Simple reason, to have more structures producing feeder heroes. While your 4 Training Camps may be devoted to four of any of the following: TC20 (2 days per training at 1 sharpening stone, 297,000 food and 100 recruits), TC11 (2 hours per training at 1,000 food and 2 recruits), TC1 (10 minutes per training at 1 practice sword, 1,600 food and 5 recruits), TC2 (*30 minutes per training at 1 adventurer’s kit, 2,000 food and 5 recruits), TC3 (2 minutes per training at 1 practice sword, 5,000 food and 5 recruits) and/or TC19 functions (*1 minute per training at 1 rugged clothes, 5,500 food and 2 recruits), you have Hero Academy at level 1 provide you with an additional source of feeder heroes, expediting the leveling process of your heroes being leveled. While Hero Academy at level 1 produces 1 star hero every minute at the cost of 2 practice swords, 2 training manuals, 5,000 food and 1 recruit, and at level 2, it spews 2 star hero every minute at the cost of 3 strong ropes, 11,000 food and 8 recruits.