Destruction of the Great Chain

"The Janissaries have raised the chain across the mouth of the Haliç (Golden Horn) and ordered a full blockade until you are caught."

- Yusuf Tazim advises fellow Assassin Ezio Auditore of the Janissaries' obstruction.

The destruction of the Great Chain was an event during the Renaissance that occurred during the year 1511 in which the Assassin, Ezio Auditore, decimated the Great Chain extending across Constantinople's Golden Horn, thus allowing him to depart from the city and journey to the Byzantine Templars' headquarters of Cappadocia.

Meeting with Yusuf Tazim
Once the Assassin Ezio Auditore learned of Cappadocia's utilization as the garrison for the remaining Byzantine Templars, he headed for the primary waterway of the city, the Golden Horn. Arriving there, he was received by Yusuf Tazim, the Master Assassin of the Turkish Assassins.

Hearing of Ezio's departure, Yusuf informed Ezio of the Great Chain being raised due to the recent murder of the Janissary Captain, Tarik Barleti. Yusuf went on to say that the Janissaries had ordered a full blockade, vowing not to lift the chain until Ezio was apprehended.

Providing Ezio with a potent bomb with fifty times the effect of an ordinary explosive, Ezio replied by telling Yusuf he understood how to utilize it. Before concluding their meeting, however, Ezio asked Yusuf to look after a close-friend and owner of a local book shop, Sofia Sartor.

Destruction and escape
"Those ships are armed. They're waiting to stop your boat. You take care of them. We will clear the docks."

- Yusuf informs Ezio of the fleet obstructing the Golden Horn.

Following his meeting with Yusuf, Ezio made his way to one of the towers the Great Chain was anchored to. Surreptitiously maneuvering past the circumspect Janissaries, Ezio placed the bomb on one of the tower's windows. Triggering the explosive, Ezio cautiously observed as the tower gave way, pulling the Great Chain along with it.

With the Great Chain lifted, Ezio scrambled across a zipline, reaching a ship docked in the Golden Horn. Meanwhile, Yusuf and his Assassins aided Ezio by eliminating the guards on the docks.

Once at the vessel, Ezio eliminated several guards and made his way to a Greek Fire situated at the bow of the ship. Utilizing the Greek Fire, Ezio destroyed the fleet obstructing his voyage from the Golden Horn. With the entire waterway engulfed in flames, Ezio dislodged himself from the Greek Fire, and carefully navigated the ignited vessels.

After an arduous venture through the incinerated ships on the waterway, Ezio ultimately arrived at his vessel. Greeted sardonically by fellow Assassin and captain of the ship Piri Reis, Ezio apologized for the impediment. With their route to Cappadocia unobstructed, the vessel began navigating the waters of the Golden Horn.

Aftermath
Subsequently after the Great Chain's destruction, the Ottoman navy was crippled from the destruction and incineration. Upon arriving at Cappadocia, Ezio made his way inside the city, meeting up with one of Tarik's Ottoman spies, Dilara.

Source

 * Assassin's Creed: Revelations