My fourth Star Citadel gallery. The game really has a few nuances, and the strategy you need to use to attack the cannon changes as you move through the levels - this was also to some extent true of the original Star Castle, particularly machines released later, which didn’t have the easily exploitable wrapping "bug".
Note that Star Citadel has two variants: a modern version, from which these screenshots are taken, and a classic version, which is designed to play as much like the original Star Castle as possible.
Click on the images to see larger versions.
The high risk inside the shields manoeuvre pays off, and I've managed to destroy the cannon.
Here we are near the beginning of a slightly later level. You can see things are hotting up in that the mines are graduating through the shield layers more quickly. Worth mentioning that they also move faster as the levels increase in difficulty.
And now we're really making progress. Shots on target, I've punched through the inner shield layer but, of course, the cannon is now firing at me. Again, on higher levels the cannon can track your ship more quickly, so opportunities to shoot at it when it's not pointing directly at you become few and far between.