Enchanted with the idea, the Prince ordered de Foix to make a book composed of twelve pieces of very hard blue marble, six inches long by four inches wide, covered, as if they were bound, with two plates of steel masked with gold.
D. Carlos always had this disguised arm at hand, ready to break the head of anyone as the fancy might take him, an extra proof of the traitorous and perverse nature of the unlucky Prince.
Besides this, there was always an arquebus at the head of his bed, and an arsenal of powder and shot hidden in his wardrobe.
After supper D. Carlos looked through the letters and papers he had prepared, and went to bed at half-past nine, leaving by the side of his bed a naked sword and a loaded arquebus, and having an unsheathed dagger under his pillow.
Meanwhile all seemed to sleep in the royal castle; nevertheless, within its walls one of the most discussed and terrible events in history was preparing.
The King kept vigil in his room, and after eleven o'clock, one by one, there arrived, cautiously, the Prince de Évoli, the Duque de Feria, the Prior D. Antonio, and Luis Quijada. These were afterwards joined by two of the King's gentlemen, D. Pedro Manuel and D. Diego de Acuña, and to all of them D. Philip spoke "as never man spoke before," according to a document of the period, and showed them the hard and terrible necessity he saw of arresting and shutting up his son Prince Carlos.
The best way of carrying this out, without scandal or dangerous resistance, was then discussed, and the King proposed his plan, which was naturally accepted. At midnight they all descended by an inside staircase, on tiptoe, in the dark, cautiously, not to arouse the guard, almost trembling, as justice has to tremble sometimes, to prevent and surprise crime.
The Duque de Feria went first, with a dark lantern in his hand; the King followed, very pale, a cuirass under his clothes, a naked sword under his arm, and an iron helmet on his head. Behind him came all the rest, with naked swords, more to inspire terror and respect than because there was need to use them. Two of the King's servants, Santoyo and Bernal, with nails and hammers, and twelve guards with their lieutenant, also came.
In the Prince's ante-room they met his two gentlemen, D. Rodrigo de Mendoza and the Conde de Lerma, who were on duty, and the King gave them orders to let no one pass.
The door of the room opened without resistance, because the King had ordered the engineer de Foix secretly to make the Prince's springs useless.