You may wish to see an introductory page with a map first.
Hisar means mountain/rock so Anadolu Hisar means the rock in Anatolia: the small fortress is located on the Bosporus, closer to Constantinople, than to the Black Sea.
Bosphorus, now called, The canal, is, according to the antients, fifteen
miles long; (..)
The narrowest part is thought to be between Rumelli Hissari (The castle
of Romelia), and Anatole Hissari (The Eastern or Asiatic castle and
consequently it must have been there, at the temple of Mercury on the
Europe side, that Darius built a bridge, in order to lead his army
against the Scythians. The castle on the Asia side was built by Bajazet
the first, when he besieged Constantinople about three hundred and
fifty years ago.
Richard Pococke - A Description of the East and Some Other Countries - 1745
Anadolu Hisar was built by Sultan Bayezit I (1389-1402) to move his troops from Anatolia to Europe safely. In particular the fortress was built to support the siege of Constantinople enforced by the Sultan in 1396. The other purpose of the fortress was to block supplies to the city from the remaining Byzantine possessions in the Black Sea and in particular from Trebizond.
View from the Bosporus
Byzantines and Ottomans were not the only players on the Bosporus.
Two other powers had strong interests in controlling the waterway: Venice and Genoa. It was a sort of strategy game where each
player pursued his own advantage and changes of alliances were very frequent with one remarkable exception: Venice and Genoa would
never stay on the same side.
Genoa had a strong economic influence in the region with a well organized trade route on the Black Sea. Relations between Genoa and the Ottomans were generally good and in 1354 the Ottomans established their permanent rule over a series of towns on the European coast of the Sea of Marmara thanks to an alliance with Genoa. For this reason the Genoese did not react to the construction of Anadolu Hisar.
Walls near the harbour
The main purpose of the fortress was to protect a small bay where a few Ottoman ships could moor and to provide a secure location for the sultan's troops while they were waiting to be ferried across the Bosporus. A small stream near the fortress provided a supply of fresh water.
Outer walls and towers, also in the image used as background for this page
The fortress was composed of an inner square castle surrounded by walls strengthened by a few towers. At that time artillery had not yet been developed, so Anadolu Hisar was designed to cope with catapults and similar siege engines.
Ottoman miniature showing (left) Rumeli Hisar and
(right) Anadolu Hisar
The threat posed by Anadolu Hisar to the passage of hostile ships was rather limited, even when towards 1450 new weaponry (cannon and bombards) was
introduced. The fortress was built on a bay and the Bosporus was at that point large
enough for ships to sail along the European side without great risk of being hit.
Sultan Mehmet II realized that to effectively block supplies to Constantinople he needed a stronger fortress on the other side of the Bosporus: Rumeli Hisar (the rock of Europe).
Fortresses of the Sultans - Introduction
Fortresses built before 1453:
1 - Anadolu Hisar
2 - Rumeli Hisar
Fortresses built after 1453 and before 1657:
3 - Kale Sultanieh
4 - Kilitbahir
Fortresses built after 1657:
5 - Seddulbahir
6 - Imbro
7 - Tenedo