Catching a king sounds like a pretty great deal, but does one do with one once you have him? Where was Francis kept while he was in the Emperor's custody? What level of confinement was he under, and under what sort of security? Was he permitted to send letters or receive visitors? Was he provided with the sort of food and finery a king was accustomed to? Were there any escape attempts or breakout attempts?
Forgive me if this question isn't framed correctly, it's my first time! Thanks in advance for any answers!
Francis was in a few different places. First he was taken to Pizzighettone for a few months, and it was during this time news of his capture trickled out to the rest of Europe. Henry VIII of England was hoping he & Charles V (his nephew) would join forces & march on Paris, where he (Henry) would be crowned king. But Charles was short on money (he would be short on money for most of his reign) & nixed that idea. Henry was not happy.
After Pizzighettone, Francis was supposed to go to a prison in Naples. However, he was pretty upset with that so he asked to be sent to Spain. On arrival in Spain he was treated as a king, but he was still a prisoner--he could go to Mass, banquets were held, tournaments, etc., but never by himself. He was imprisoned in the Alcázar. This is how the Duc de Saint-Simon described it:
"The room was not big, and it had only one door, through which one entered. It was made a little larger by an embrasure on the right as one came in, facing the window. The latter was wide enough to give some daylight, it was glazed and could be opened, but it had a double iron grill, strong and stiff, which was welded into the wall...There was enough room for chairs, coffers, a few tables and a bed."
Peace talks began in July 1525, with Francis's mother (Louise of Savoy) & sister (Margaret of Alençon) attending; the Treaty of Madrid was signed in January 1526. Francis was released on 17 March 1526, however he was required to hand over his two boys (the younger of the two became Henri II on the death of Francis) as hostages for Francis's good behavior. The children remained there for four years in conditions that worsened every year, while his father took the slow route back to Paris so the country could celebrate his return.
TL;DR: while he was prisoner Francis was treated as his position allowed & was used as a bargaining chip, his sons, however, were not so lucky.
Source: "Four Princes" by John Julius Norwich, 2016 (I'm currently halfway through)