This is the Yıldız Hamidiye Mosque, built on the orders of Abdülhamid II (1842-1918, reigned 1876-1909). It was constructed in 1884-1886, architect Sarkis Balyan. The Balyan family (Armenian: Պալեաններ) was a Armenian dynasty of famous Ottoman Imperial architects. For five generations in the 18th and 19th centuries, they designed and constructed numerous major buildings, including palaces, kiosks, mosques, churches, and various public buildings, mostly in Constantinople, present-day Istanbul. The nine well-known members of the family served six sultans in the course of almost a century and were responsible for the westernization of the architecture of the then-capital. Sarkis Balyan (1835–1899) was part of the fourth generation; amongst his most notable works one can mention (amongst many others): the Beylerbeyi Palace (with his father Garabet Balyan), the Malta Pavilion (in the garden of Yıldız Palace), the Çırağan Palace, the Pertevniyal Valide Sultan Camii in the Aksaray district of Istanbul (with his brother Hagop Balyan), the Dolmabahçe Clock Tower.
The guidebook Strolling through Istanbul expresses horror at the mosque: “At the northeastern corner of the [Yıldız] gardens, just outside the enclosure, is the ugly Hamidiye Mosque …” I had never entered it before, and disagree. Its exterior is not very impressive, but the interior, for me, was a joy. A number of high-rising columns make for a spacious central room, and the decoration is almost wild in its exuberance.
A notice outside, in as so often garbled English, states “Ceilings, domes, niches, walls and maksurs are covered with Maghreb (Nport Africa) inspired meringues at a density to perceive the relevance of the palace and mosque. Small and high dome of the mosque is seated on a polygonal pulley with six windows. Neogothic windows and stalactite sequence add a different atmosphere to mosque pulley.“ And so forth. I think the word “kasnak” (that can be translated as “pulley”) was Google-translated. As I found it can also mean “tambour” which according to the Wikipedia is “In classical architecture, a tambour (Fr.: "drum") is the inverted bell of the Corinthian capital around which are carved acanthus leaves for decoration. The term also applies to the wall of a circular structure, whether on the ground or raised aloft on pendentives and carrying a dome (also known as a tholobate), and to the drum-shaped segments of a column, which is built up in several courses.“
In the same poor language it is explained there are two pavilions (hünkar mahfili) in the mosque reserved for the sultan, two, because an assassination attempt might fail as it would be unclear which of the two contained the sultan. I killed myself almost when stepping back to better take the picture of the two, and found I stepped into the stairwell. I reacted quickly and survived.