Vareth Dar was a Cardassian Alliance surveillance sensor station in the mirror universe. The station, which was manned by Cardassian personnel and equipped with powerful weaponry and strong shields, was located in the Cuellar star system.
In 2375, the Terran Rebellion identified Vareth Dar (known to them by the nickname "the Watchtower") as a target for destruction, as it enabled the Alliance to monitor ship movements for forty light years, including around Terok Nor and the Betreka Nebula. Vareth Dar was later destroyed in an attack by the Defiant and the Capital Gain. (DS9 - Mirror Universe novel: Saturn's Children)