Entrance to the Second Court is through the Middle Gate, or the Gate of Salutation that was built by Mehmet II and was never altered except in details. The Middle Gate is larger and more monumental than the first gate thus providing the sense that one was entering a sacred space; this monumental double-towered gate appropriated a Roman palatial motif symbolic of sovereignty and imperial power.  This gate seems to have been modeled on the Gate of St. Barbara (Cannon Gate), which was the royal seaside entrance to the palace gardens from the Byzantine city wall on the shore. It was at this gate that a respectful silence was imposed and only the sultan could ride his horse beyond this point as entrance to this gate was restricted to only those with official business.The inscription, 'Babu's-selam' (Gate of Salutation or Gate of Peace) at once decorates the gates and infers a peaceful image of paradise that visitors would encounter upon crossing the threshold into the second court. A comparison to the gates into the Garden of Eden was successfully achieved through various inscriptions from the Quran embellished in the inner façade of the gate showcasing gazelles, peacocks, fountains and trees.  Interestingly, above the vestibule was a masjid for gatekeepers; its ground floor flanked by dormitories where the gatekeepers slept. The two dormitories flanking the vestibule were connected at the right to a third hall that functioned as a state prison.

by Aliza Sovani, Harvard Graduate School of Design MLA'15/Archnet Content Contributor, 19 September 2015

Source:

Necipoglu, Gulru. “Architecture, Ceremonial and Power. The Topkapi Palace in the Fifteenth and Sixteenth Centuries.” New York: The Architectural History Foundation, Inc, 1991. 50 - 52