“The Duke [i.e., of Mayenne, leading the forces of the Catholic League against Henri of Navarre] … was also overtaken with new trouble, at the liberty of his Nephew Charles, Duke of Guise … That Prince, since the death of his Father [Henri, Duke of Guise], had always been kept prisoner; nor, though his freedom had been much treated of, had any attempt ever succeeded, and the King (Henri IV) had always stiffly denied to change him for any body, alleging, That he was not a prisoner of War, but of Justice: Nor … had the Duke of Mayenne ever cared much to get his liberty; foreseeing that his freedom would endanger the division of his party, by reason of the dependence that many would have upon him, in respect of the memory of his Father, … and that the common people would willingly concur to exalt him: so that if he [i.e., Charles] would not acknowledge his [Mayenne’s] superiority, but should attempt to put himself in the place long held by his Father and Grandfather, the League was without doubt like to be divided and disunited …
…But now, whether the King (as some believed) foreseeing the same, had underhand given way to his enlargement, or that the Sieur de la Chastre, an old servant and dependent of his [Charles’] father’s … had prosperously procured it; certain it is, that having plotted, and agreed that a Lackey and a Valet de Chambre with a very swift Horse, sent by la Chastre, should stay for him in the fields under the Castle of Tours, in which he was kept prisoner, he upon the fifteenth day of August, being risen from Table about noon, and having afterward shut himself up in his Chamber to take his rest, while the Guards that kept him, and his other servants, entertained themselves merrily eating and drinking, he having locked them all dexterously into the room where they were at dinner, went up to the top of a Tower that stood toward the field, and with a ladder of silk, which had been secretly sent him in a Pie, let himself down the wall, with exceeding great danger; and being come safe to the ground, ran along the Riverside of the Loire towards the fields, where he found the horse and those that expected him; and with infinite speed galloped to find the Baron de la Maison, son to the Sieur de la Chastre,
[A studiedly slow chase was mounted] “which confirmed the jealousie some had, that the King had secretly commanded he should be permitted to escape, since that all those dayes, Letters and Messages were without restraint suffered to come to him, and Presents to be sent, among which was the Pie with the Silk Ladder in it, without which his escape could not have been effected.”
My source is Arrigo Caterino Davila, The history of the civil wars of France written in Italian, by H.C. Davila; translated out of the original (1678, p.510). Subsequently, the dextrous King Henri IV, by then a sort of Catholic, paid Charles off very handsomely, to keep him sweet. A better escape attempt than Charles I's. If it was a put-up job, perhaps King Henry thought it was a win-win for him, if Charles (Duke of Guise) broke his neck, or survived to divide loyalties in the Catholic League.