The tragic story of prince Cem is worth telling:
He was the youngest son of Mehmed II and half-brother of Bayezid II. At the death of Mehmed the Conqueror (May 3, 1481) Bayezid was the governor of Amasya and Cem of Konya. The grand vizier Karamanlı Mehmet Paşa – believing to be fulfiling the wishes of the deceased Sultan – attempted to arrange a situation (Konya being closer to Istanbul than Amasya) whereby the younger son Cem would arrive prior to his older sibling and be able to claim the throne. The grand vizier's plan were discovered by the Janissary corps, who supported Bayezid over Cem; they rebelled, lynched the grand vizier and began a widespread rioting. Understanding the danger of the situation, former grand vizier Ishak Paşa beseeched Bayezid to arrive with haste; which Bayezid did on May 21 and was declared Sultan.
Only six days later, Cem captured the city of Inegöl with an army of 4,000. Sultan Bayezid sent his army under vizier Ayas Paşa to kill his brother. On May 28, Cem had defeated Bayezid's army and declared himself Sultan of Anatolia, establishing his capital at Bursa. He proposed dividing the empire between them, leaving Bayezid only Europe. Bayezid furiously rejected the proposal. Declaring "between rulers there is no kinship," he marched on Bursa. The decisive battle between the two rivals took place near the town of Yenişehir. Cem lost and fled with his family to Mamluk Cairo.
There Cem received an offer from his brother: one million akçes (the Ottoman currency) to stop competing for the throne. Cem rejected the offer, and in the following year he launched a campaign in Anatolia. He besieged Konya, but was soon forced to withdraw. He intended to give it all up and return to Cairo, but all of the roads to Egypt were under Bayezid's control. So, Cem and a few followers asked protection of the Spanish captain of Bodrum Castle, a stronghold of the Knights of St. John. Their grand master Pierre d'Aubusson then invited Cem to Rhodes, where he was received with honour on June 29. In return for the overthrow of the new sultan Bayezid, Prince Cem offered perpetual peace between the Ottoman Empire and Christendom. However, the sultan offered the Knights a large amount to keep Cem captive; the Knights took the money and betrayed Cem, who became a well-treated prisoner. Afterwards, Cem was sent to the castle of Pierre d'Aubusson in France. Sultan Bayezid sent a messenger and requested Cem to be kept there; he agreed to make an annual payment in gold for his brother's expenses.
In 1489 Cem was transferred to the custody of Pope Innocent VIII, who unsuccessfully attempted to use Cem to begin a new crusade. The Pope also tried to convert Cem to Christianity, without success. Cem's presence in Rome was useful nevertheless: whenever Bayezid intended to launch a military campaign against Christian nations of the Balkans, the Pope would threaten to release his brother. In exchange for maintaining the custody of Cem, Bayezid paid Innocent VIII 120,000 crowns (at the time, equal to all other annual sources of papal revenue combined), a relic of the Holy Lance (which allegedly had pierced the side of Christ), one-hundred Moorish slaves, and an annual fee of 45,000 ducats.
Cem died in Capua (February 1495), while on a military expedition to conquer Naples under the command of King Charles VIII of France. Sultan Bayezid declared national mourning for three days. He also requested to have Cem's body for a Muslim funeral, but it was not until four years after Cem's death that his body was finally brought to the Ottoman lands because of attempts to receive more gold for Cem's corpse. He was buried in Bursa, in the tomb of his oldest half-brother Mustafa Çelebi.

Correspondent: J.M.Criel, Antwerpen.
Source: (amongst others) Wikipedia.

Condensed from a notice at the site: Built for Shehzade Mustafa, son of Mehmet the Conqueror. Mustafa died in 1474, his remains after some travels were brought to Bursa, to be buried in the mausoleum of an uncle, until in 1479 this mausoleum was built for him and he was buried here. Another son of Mehmet, Cem Sultan, died in 1499, was buried in the same mausoleum, which led to it today being called Cem Sultan Mausoleum.
Its decoration is part with Iznik Tiles (turquoise and dark blue hexagons with in their centre and at the edges zoomorphic motifs), part with hand-drawn ornaments which for a long time were covered with 19th century baroque ornaments, but were uncovered during a 2013 restoration.