Ward Gates appeared in Blood Omen 2. Following Kain's defeat by the Sarafan Lord, two hundred years after the events of Blood Omen, the Sarafan order's power had grown again. Confronted with these well armed, modern Sarafan - and divided by the collapse of their army - the vampires' post-Blood Omen renaissance was coming to an end, and they were facing extinction once more.
The Ward Gates were devices used by the Sarafan in their ceaseless war against the vampires. Powered by (and likely formed from) green Glyph energy, they prevented vampires moving freely through Meridian's streets, although they would not stop humans from passing through. If a vampire touched an activated Ward Gate, it would repel them and burn them as if it was a wall of fire.
Kain first saw a Ward Gate in the Slums, while Umah was training him. Later Kain was separated from her when a Glyph Guard turned on a Ward Gate between them. Kain had to turn off or avoid many Ward Gates while he was travelling through Meridian.