Istanbul, formerly known as Costantinople, is the largest city in Turkey.
- "How was the ride from Istanbul?"
- ―Guest House Security System
Istanbul was the word chosen as part of the sign used by the Guest House Security Protocol to allow entrance to the people that approached the compound.
Phil Coulson, Grant Ward, Leo Fitz and John Garrett approached the gates of the compound hoping that the medical staff of the facility could help Skye to recover from the critical injuries she sustained while trying to apprehend Ian Quinn in Italy. As none of them knew the countersign and it wasn't found in any of S.H.I.E.L.D.'s directive protocols, Coulson tried to reason with the people inside, but he obtained no response and decided to hack their way inside the facility.