Complex sentence: Although deeply indebted, in fact, under the Council of Castile administration because of his young age, to avoid money mismanagements, the estate of Osuna was only second by total wealth in Spain (after the Duke of Medina Sidonia).
Split into simpler sentences: Although deeply indebted, the estate of Osuna, came under the Council of Castile administration because of his young age, to avoid money mismanagements. The estate of Osuna was only second by total wealth in Spain (after the Duke of Medina Sidonia).