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I completed it in 6 days because, starting from the fourth day, the hero rode to the Academy with a medium-speed army. However, it is possible to capture this Castle as early as the 4th day if you take the Boots of Nomad from the Druids and move only with a fast army. The remaining Castles can be captured with fast movement on the same fourth day. Unfortunately, there wasn't enough time to develop this tactic.