And it told all about what Columbus had suffered, and was, so full of sorrow and humbleness and yet of pride in what he had been able to do, even though he had been disgraced, that Queen Isabella (who was really a friend to Columbus in spite of her dissatisfaction with the things he sometimes did) became very angry at the way he had been treated.
She took the letter to King Ferdinand, and at once both the king and the queen hastened to send a messenger to Columbus telling him how angry and sorry they were that Bobadilla should have dared to treat their good friend the Admiral so.

They ordered his immediate release from imprisonment; they sent him a present of five thousand dollars and asked him to come to court at once.
On the seventeenth of December, 1500, Columbus came to the court at Granada in the beautiful palace of the Alhambra.

He rode on a mule.

At that time, in Spain, people were not allowed to ride on mules, because if they did the Spanish horses would not be bought and sold, as mules were so much cheaper and were easier to ride.

But Columbus was sick and it hurt him to ride horseback, while he could be fairly comfortable on an easy-going mule.