Your final push in this chapter is to breach the outer walls of the Citadel. This involves destroying several Combine generators or force field emitters. You'll face multiple Striders and Combine Gunships in rapid succession. Keep moving, use cover, and prioritize targets. Your resistance allies will help, but the main burden falls on you. Once the outer defenses are down, you'll gain access to the Citadel itself, marking the end of the urban combat and the beginning of the final assault.
Half-Life 2
The Citadel's Outer Walls — Half-Life 2 Walkthrough
Breach the Citadel's outer walls in Half-Life 2. Destroy Combine generators and fight Striders and Gunships to gain entry to the final assault.