Star Citadel Tips & Tricks
In early versions of the original Star Castle arcade machine you could take advantage of the screen wraparound to zip back and forth between the top and bottom of the screen to progressively drill through the shield rings, and so amass a huge score. Later machines fix this loophole and, you'll find, that it also doesn't work so well in Star Citadel. The basic principle still holds true though: try to go deep when destroying sections of the shield rings. Firstly, because you won't be able to shoot the central turret until you've penetrated all rings. Secondly, because if you destroy all the segments within a shield ring, the turret will simply regenerate a new one.
Keep on the move. You won't notice this so much in lower levels but, as the game progresses, camping becomes an increasingly poor strategy. You become easy pickings, both for the faster moving mines, and the central turret's fire, which increases in size and speed as the levels increase. Unlike Space Rawks! (c.f., Asteroids), where you want to remain around the middle of the screen, with Star Citadel you want to move around as much as possible. Wrapping around the edge of the screen is an easy way to avoid cannon fire (and cannon lock on) on higher levels. Moving is particularly important in the classic game because your ship is harder to maneouvre, and your firepower is compromised.
Take advantage of the fact that your ship will bounce off the shield rings. You can use this to quickly change, or reverse, direction.
Unlike the original version of Star Castle, in Star Citadel you can fly between the shield rings (not in classic mode though). This can be kind of nifty, in that you can dodge mines, or fly in for a swooping attack and then skim through the rings to shoot out the other side. On the other hand you can become trapped. Not to mention you become extremely vulnerable to cannon fire if you're between the two innermost rings. You might get a lucky shot in to destroy the cannon, but it'll most likely be an ai uchi (simultaneous strike) where you end up getting killed at the same time.
If you collide with the central turret you'll be destroyed but it won't be, so no kamikaze runs.
If you take too long to clear a level it gets faster - in fact it gets faster at regular (and shorter) intervals. This can make it extremely challenging to clear a level. Under these circumstances it can be beneficial to take an ai uchi - i.e., kill the cannon as it kills you - in order to progress to the next level, which will then be somewhat easier than what you just experienced.
If you shoot whilst accelerating from standstill, you can build a clump of bullets. Correctly applied you can use this clump to punch deep into the cannon's shield rings. On lower levels this clump can even penetrate right through to hit the cannon itself.