In Quake 4, the primary focus of act 1 the Strogg Air Defense Cannon is a large gun designed to destroy enemy spacecraft or other large airborne targets. It is captured by Rhino Squad after the pacification of the Strogg hangars pinning down an advance. Before it's destruction on the behalf of Rhodes it is used by Kane to destroy a large locked door blocking the way to the Hannibal's landing site, in the process taking out a handfull of Strogg infantry. After the destruction of the cannon, USS Hannibal is finally safe to touch down and the act 1 ends.
Quake 4 - Walkthrough - Part 3 720p
A walkthrough of clearing the Strogg cannon bunker