At the Nebula Boundary between the Federation and Caesar, there was a huge man-made military fortress. This fortress was well-known throughout the galaxy. It was known as the fortress that Caesar couldn't destroy.
Base Nebula was in possession of the most up-to-date military technology. It was built with the intent of it being an easy to hold but hard to attack movable base. In the face of a moveable and impenetrable base, Caesar couldn't do anything to it other than just passively defending from it.
Of course, besides being able to move and having the ability to turn itself into a porcupine with powerful firearms all around it, Base Nebula was filled with powerful people too.
Base Nebula only had mechas as its weapons. It had no need for other weapons.
