The citadel (Al Qalaa) was built in the 12th century by Saladin, the man who helped throwing out the crusaders from Jerusalem. The citadel was part of his defense against the crusaders, who were looking for revenge. Saladin managed to stretch the boundaries of is empire finally from Yemen to Tunisia.
In 1798, Napoleon took over the citadel but was thrown out not long after that by the Ottoman Muhammed Ali. Some of his children and grandchildren lived here, until it became a military garrison. In the second World War, the British used the Citadel to house their soldiers, which were replaced by Egyptian soldiers in 1952, some of which still are there.
Inside the citadel, there used to be Mamluk palaces, which then were destroyed by Muhammed Ali and replaced by a mosque carrying his name, which was built in the beginning of the 19th century. He was from Ottoman origin and as you can see, the mosque is of Ottoman style and looks a little like the blue mosque in Istanbul, on the picture next to it.