ICO: Castle in the Mist

The Castle of the Mist calls for a sacrifice: a horned child, born once a generation. When, on a single night in his thirteenth year, Ico's horns grew long and curved, he knew his time had come. But why does the Castle of the Mist demnd this offering, andcan the castle keep Ico's destiny from intertwining with that of the girl imprisioned within his walls?