You can send a unit into the citadel and pillage the citadel (generates a bit of gold, heals the unit +25 HP and makes the citadel inoperative (no more -30 HP damage effect, until Harald repairs the citadel).
Yes, you can claim the citadel for yourself if you conquer the city that "owns" the citadel tile. In that case, the citadel becomes your citadel, and enemy civs will experience the -30 HP damage effect. Alternatively, you can use your own Great General to "citadel-bomb" the citadel tile, which will also make that citadel your citadel.
EDIT: Derp, I should remember to read the past page of a thread before responding. @beetle,
Yes, you can claim the citadel for yourself if you conquer the city that "owns" the citadel tile. In that case, the citadel becomes your citadel, and enemy civs will experience the -30 HP damage effect. Alternatively, you can use your own Great General to "citadel-bomb" the citadel tile, which will also make that citadel your citadel.
EDIT: Derp, I should remember to read the past page of a thread before responding. @beetle,