The goal of the Star Castle is to destroy the enemy cannon, which is located in the center of three concentric, rotating energy shield rings, while avoiding or destroying "mines" - enemies that emerge from the core, pass through the energy rings, and then hit the player's ship. They can stick to the shield if the player maneuvers in such a way that there is a ring between the ship and the mines. The player-controlled spaceship can spin, throw forward, and fire small projectiles. The cannon's shields are made up of twelve sections each, and each section takes two hits to destroy. When a section is breached, the rings underneath are exposed to fire.
Once the inner ring has been breached, the center weapon becomes vulnerable to attack from the player. However, the player is also more vulnerable at this point, as with the shield rings removed, the pistol can fire a large projectile that hisses with white noise. In addition, the central core constantly tracks the player's movement. If the player manages to hit the cannon, it explodes violently, destroying the remnants of the shield rings, and the player gains an additional ship. The next level starts with new weapons and fully restored shield rings, with increasing difficulty (mines move faster, rings spin faster, and the core tracks the player faster).
If the player completely destroys the outermost defensive ring, the cannon will create a new one. The middle ring expands to replace the lost outer ring, the inner ring replaces the middle one, and a new nucleus emerges from the nucleus to become the inner ring. Therefore, in order to penetrate the cannon's defenses, the player must be careful not to completely erase the outer ring.
Three homing mines will destroy the player's ship on contact. The mines can be destroyed, but they are very small and difficult to shoot at, and the player does not receive points for destroying them. The mines regenerate when the shield rings regenerate (in some variants, the three mines are constantly churned so that a new mine is respawned from the core as soon as one of them is destroyed). As the player progresses through levels, the mines get faster and faster, forcing the player to keep moving to avoid them.