To tell the truth, the Citadella itself is not so interesting. This is an impressive place for panoramic photos that can be shoot here - of the city, of the Danube, and the rest of what is around.
Want romance? - Get here at night, or at sunset, and you will be rewarded by a romantic and pretty view of the city at twilight and the twinkling lights.
Next to the Citadella is the famous Freedom Monument, that was established by the Communists as a memorial for Soviet soldiers, who freed Hungary from Nazi rule. Go see it yourself - it is beautiful and well lit at night.