Complex sentence: General Yermolov constructed a major fort at Vnezapnaya on the nearby Koysu in 1820, which was used to suppress the Endirey slave fair support the local shamkhal against the Kumyk nobility, and formed the staging point for the 1839 phase of the Caucasian War.
Split into simpler sentences: General Yermolov constructed a major fort at Vnezapnaya on the nearby Koysu in 1820, which was used to suppress the Endirey slave fair and to support the local shamkhal against the Kumyk nobility. The fort also formed the staging point for the 1839 phase of the Caucasian War.