Across from Hagia Sophia, the Imperial Gate was the first and most accessible of the three main gates of the Topkapi Palace which was designed to be experienced sequentially from public to private access. The Imperial Gate (Bâb-i Hümâyûn) and outer wall were added to the fortress in Ramadan 883 AH, that is between November and December 1478, after the buildings and gardens had been finished.
The Imperial Gate is the first entrance into the palace and thus separates the palace from the city. Additionally, this gate includes Celi-Sülüs style calligraphy (Sülüs: proportioned calligraphic script type; Celi: meaning clear, conspicuous to describe large, monumental versions of script during Ottoman times) with an inscription by Ali bin Yahya Sofi with a date of 1478/883AH placed above the doors. Considering that the signatures of the Sultan Mehmet II and Abdulaziz are below the inscriptions could mean that the door may have been repaired a number of times. On each side of the gate are small rooms for gatekeepers and in 1866/1282AH, a fire destroyed the manor style flat used as Beytül mâl (Kapı arası hazinesi or to keep the goods are not yet taken to the Sultan's treasury) which rested above the door and was used by Fatih for himself.
Necipoglu, Gulru. "Architecture, Ceremonial and Power. The Topkapi Palace in the Fifteenth and Sixteenth Centuries." New York: The Architectural History Foundation, Inc, 1991. 6-8,40.
"Sections: Touring the Topkapi Palace." Topkapi Palace. Republic of Turkey Ministry of Culture and Tourism. 2006. Web. Accessed January 14, 2015.