This is where things are getting a bit more serious. This is the beginning of a level but you can see the cannon's already managed to get off a very accurately targeted shot, and a mine has already spawned and is heading straight for my ship. My wild firing is doing absolutely nothing.
Yep, totally too late, and the fact I managed to take out the mine is absolutely no consolation. Again, it's probably not apparent from these shots, but in Star Citadel the energy projectile fired by the cannon has a very primitive homing ability that becomes more pronounced on higher levels. In Star Castle, by contrast, whilst the energy projectile gets faster (and, I believe, larger), it always flys straight.
I'm back in the game with renewed vigour and mount a deadly scything attack on the cannon, which penetrates all three shield layers in one go, leading to the immediate destruction of the cannon. Nice. Only problem is that energy projectile.