The remains of the fortress can be visited
Rumeli Hisarı Castle

The Rumeli Hisarı Fortress, an Ottoman structure, rises on the European side of the Bosphorus in Istanbul. It was built at the narrowest point of the strait.
History+
In 1452, Sultan Mehmed II ordered the construction of the Rumeli Hisarı Fortress. This occurred nine months before the conquest of Constantinople and was a vital step in the preparations for the siege. The construction of the complex, intended to control the Bosphorus at its 700-meter width, was completed by 3000 Ottomans in four months. After the capture of Constantinople, the fortress temporarily served as a prison. Until 1830, the three main towers, named after the viziers Saruca Pasha, Halil Pasha, and Zağanos Pasha, were still topped with lead spires.
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This description draws on the facts documented in the linked Wikipedia article; Wikipedia content is licensed under CC BY-SA 4.0. Wording and editing: Mobile History.
Location: 41.0844, 29.0561