From The Sultans, by Noel Barber (Simon & Schuster, 1973), pp. 77-81:
Mahomet [Mehmed III]… was the last sultan ever to be trusted with liberty during the lifetime of his predecessor. And his nineteen brothers were the last to be strangled under the law of fratricide. (This did not prevent some future heirs to the Sultan from living in terror of the bowstring, often with reason.)
Not long afterwards [his Venetian mother, Safiye Sultan Sofia Bellicui] Baffo was strangled in her bed; her death did not mark the end of the harem rule, but another influence dominated the lives of the princes who followed Mahomet. It was a fate in many ways more grim than death itself. To make certain they would never become involved in plots against the reigning Sultan, any possible heirs were immured in a building in the Grand Seraglio. It was called the Kafes. Its literal translation is ‘The Cage’.
The Kafes was not a barred cage in the accepted sense of the word, but it was most certainly bolted. It consisted of a two-storied grey building tucked away behind a high wall in the heart of the Grand Seraglio, almost opposite the rooms of the first Sultana. It had handsome courtyards and gardens, and its tiled walls were among the most beautiful in the Seraglio. There was, however, one sinister note. There were no windows on the ground floor, though those on the second floor looked out to sea.
For the next two centuries heirs to the throne were immured, sometimes from the age of two, until they were either called to the throne, or their miserable lives were mercifully ended with the bowstring. One heir was to remain nearly fifty years without ever leaving the building, and when he emerged to be proclaimed Sultan he had all but lost the power of speech. The princes’ only companions were deaf mutes [who also served as the Sultan’s assassins] unable to give news of the outside world, and a modest harem of concubines, the only living creatures to who they could talk. Once inside, the odalisques suffered the fate of their masters. They never left the Cage unless one carelessly became pregnant, in which case she was immediately drowned. This happened very rarely for great care was taken to make these women barren – either by the removal of their ovaries or by the use of pessaries (made up by the Seraglio doctors from a bewildering assortment of ingredients, including musk, amber, aloes, cardamom, ginger, pepper and cloves.)
Sultan Ahmed I, who succeeded Mahomet in 1603, founded the cages because he rebelled against the barbaric custom of fratricide; perhaps he was ever proud of discovering such a humane method of guarding his brothers’ lives. But it is not difficult to imagine the debasing effects of years of solitary confinement on men who were expected to take up the reins of office at a moment’s notice after half a lifetime in which their minds and bodies had vegetated. As N. M. Penzer, a leading authority on the harem, wrote, ‘The Kafes has been the scene of of more wanton cruelty, misery and bloodshed than any palace room in the whole of Europe. To its institution are due the weakness, vices and imbecility of so many of the Sultans and, to a large extent, the gradual decay and fall of the Ottoman Empire.’ …
During Ahmed’s reign Mustafa, who succeeded him, spent more than ten years in the Cage, providing the first terrible evidence of its effect on human beings, as each succeeding sultan seemed more made, avaricious, debauched and besotted than his predecessor. By the time Mustafa I became Sultan he was completely demented. He appointed to favourite pages – scarcely out of their infancy – to be Governors of Cairo and Damascus. He dismissed a high-ranking officer so that he could offer the post to a peasant who gave him a drink of water when hunting. He clapped the French Ambassodor in the Castle of Seven Towers on the flimsiest pretext. After three months he was deposed – very politely. A five-day hunting trip was arranged for his enjoyment, and when he returned he was no longer Sultan. He went back to the Cage. His nephew Osman II, who succeeded him in 1618 … was even madder. His favorite pastime was archery, but he only enjoyed the sport when using live targets. Prisoners of war were considered fair game for the Sultan, but when there was an insufficient supply. After four years of misrule – or, rather, no rule at all – the Janissaries decided he must go…. It was the first regicide in Ottoman history.