The Augustgrad gates were built on the outskirts of the city. They were security gates that lead into the heart of Augustgrad.
In 2505, when a resurgent Zerg Swarm invaded Korhal, Sarah Kerrigan and her forces, in accordance to Valerian Mengsk's request to land the swarm outside Augustgrad, destroyed the gates, allowing the Swarm to flood into the city.
Augustgrad gates appear as the bonus objective in "Planetfall," where the player is tasked to destroy three of them. Destroying the gates will cause zerg forces to spawn and flood through them, but provides no gameplay benefit to the player in "Planetfall" or future missions.