Only rebuilt walls and foundations remain. There's a couple buildings and some historical markers to pour over. Also a good little history museum. There's some great views of the city from the top. There's a nice information center with a sweet lady who works there. I asked her about the controversy of whether there was actually a castle here or not. Her Japanese was way above my head. I liked chatting with her for some reason.

Went here December 2017 and 2018 during winter and there's not much to see but when I went last April, it was such a nice place for hanami and cherry blossoms viewing. It has lots of cherry blossoms in the vicinity. There is also illumination of the cherry blossom trees during night time

Fukuoka Castle Park is a great place for a walk around the huge walls & foundations. Few structures remain and not much has been reconstructed - there's far more to see at Kanazawa Castle Park, for example. Best place in Fukuoka for cherry blossoms.