A maximum of two champions can be trained in this building. The first training slot is free but the second slot can be unlocked for 50 diamonds. Training a champion does not cost any resources and takes 24 hours (or less with guild military bonuses).
When this building is upgraded, all trained champions within it by that time will turn into unattached champions to make room for production of attached champions of the newer age.
ADVICE for Champion's Retreat: As you advance in age, your champion's retreat will become less valuable without an upgrade. Once you upgrade your retreat, you cannot lower the age again in the future. This is important if you ever participate in Guild vs Guild. Having Champion's Retreats of lower ages can be very useful for setting Defense Armies and for fighting, so consider using a Store Item to put the building back into your inventory instead of upgrading it to a higher era.