


It's possible, but very unlikely that you can manage to citadel a cruiser in a destroyer - it typically happens when you're firing at a very close range. If you're in a destroyer, don't bother trying to citadel a battleship - your AP shells will likely just bounce.

It is possible, just unlikely, to manage to citadel a battleship as a cruiser - your guns may not have enough penetration to break through a battleship's side armour. If you're in a cruiser, you are capable of hitting other cruisers' citadels with AP. If you're in a battleship, you are capable of hitting other battleships' citadels with AP at all ranges, and cruiser citadels with AP at all ranges (or HE if they're lightly armoured enough).
