Rumeli Fortress, Istanbul

Rumeli Fortress, Istanbul, originally uploaded by - di.

At 660 metres, the width of the Bosphorus Strait was narrowest at the point where this fortress was built in 1452. Actually Rumeli Hisari is considered one of the world's most beautiful pieces of military architecture.

It was built by Mehmed II, a then 21 year old leader who decided to conquer what he considered to be the politically, militarily and socially decaying Byzantium and so he built two forts, one on either side of the Bosphorus Strait. The Rumeli Hisari on the European side and the Anadolu Hisari on the Asian side of the strait, with the intention of controlling the traffic of foreign ships sailing to and from the Black Sea.

It was effective and in November 1452, a Venetian line of battleship under the direction of Antonio Rizo was sunk for having disregarded the signals given by the fort.
Source: 'A Guide to Istanbul' by Çelik Gülersoy