Kafes, literally "the cage", was the part of the Imperial Harem of the Ottoman Palace where possible successors to the throne were kept under a form of house-arrest and constant surveillance by the palace guards.
The early history of the Ottoman Empire is littered with succession wars between rival sons of the deceased sultan. It was common for a new sultan to have his brothers killed, including infants, sometimes dozens of them at once. This practice reduced the number of claimants to the throne, leading to several occasions where the Ottoman line seemed destined to end. The confinement of heirs provided security for an incumbent sultan and continuity of the dynasty.
When Ahmet I died in 1617, his eldest son was only 13 years old, and for the first time in 14 generations, the Imperial Council altered succession so that the late sultan's brother acceded to the throne as Mustafa I, aged 25 years. He was deposed (for the first time) the following year and became the first inmate of the Kafes although he and other princes throughout the preceding generations had been sequestered in various other places of comfortable confinement.
It feels like I'll never be the same
I want to live this way
Empowered to the end
To change the way you see
It feels like I'll try to find a way
To always live this way
The power in my hand
Will change the way you feel
Try fixing this, you'll never find a way