From the manual of the game:
In Stronghold you control a small spacecraft which you pilot on an assault against a well defensed asteroid. Your goal is to neutralize the asteroid's defenses by destroying the Command Crawler that controls them.
First you must survive a barrage of drones launched from the surface. As you maneuver to blast through the shields that protect the Command Crawler, interceptors home in on your ship and patrollers hover above the shields. When the shields are breached, the Command Crawler itself begins to return your fire.
If you can destroy the Command Crawler, a new and better defended asteroid awaits.