The Sentinel was a Golan II Space Defense SpaceGun used by the Rebel Alliance, located at Kothlis II. It was constructed to ensure that the main facility on Kothlis II was well-defended from Imperial attacks.
During the Battle of Kothlis, the Rebel facilities were attacked by the Executor and the Avenger, which were sent by the Galactic Empire to recover a stolen computer that contained the Death Star II plans. Sentinel was the closer target and was bombarded by TIE/sa bombers from the Avenger. The base was finally destroyed from concentrated enemy fire, leaving Kothlis II vulnerable.