Final answer:
The fortress that Peter took from the Turks but was forced to return to them in exchange for his freedom was the Fortress of Azov.
Step-by-step explanation:
In 1695, during the reign of Peter the Great of Russia, the fortress that he took from the Turks but was forced to return to them in exchange for his freedom was the Fortress of Azov. The fortress that Peter the Great took from the Turks but was later forced to return to them in exchange for his freedom was Azov (a). This event occurred during the Russo-Turkish War of 1695-1696. After successfully capturing Azov in 1696, Peter faced challenges, including a lack of naval support and the Ottoman threat. Consequently, the Treaty of Karlowitz (1699) forced him to cede Azov back to the Ottoman Empire. This setback influenced Peter's subsequent military and naval reforms as he recognized the need to strengthen Russia's military capabilities, laying the groundwork for his broader efforts to modernize the country in the early 18th century.