A castle that had been built by Bayezid I "The Thunderbolt" (Ottoman Sultan) in 1391 by the narrowest place of Istanbul Strait (BOSPHORUS) in order to facilitate the conquering of the city.
In English it is known as Anatolian Castle. It is very close to the second bridge that connects Asia and Europe in Istanbul. By car, it takes 3 minutes to go to Asian side of the city from this castle.
Anadoluhisari is also a name of a town where this castle is in. It has really clear air, and green areas which you cannot easily find rest of Istanbul. Marmara University's sports faculty and business administration (only in German) are here.