From my Favorite Easy Demon Ranking video:

It's your typical Subwoofer purple castle level that has a spooktacular song made for NCS x GD album. The gameplay is pretty fun the second time you play it, as the first time... yeah, I can't imagine it being fun because of the sideways ship at the end. The decoration really sold it for me though, I love the way the level looks, which is unironically a given as aside from space themed levels, castle themed levels really suit my fancy. And also, the artstyle. I really like Subwoofer's artstyle that he puts into his levels, it feels a lot GD like if that makes any sense. Overall though, Citadel is quite the level and one that I personally enjoy playing, but would not recommend it to people unless you can do sideways gameplay. And once again, the song is a banger. Boom Kitty and Waterflame at their finest people.