It's a puzzle platformer.
In this game, you play as Halli-Valli, a purple creature who discovers a castle. He also finds Huuma-Tuuma (and he is not a ghost) and together they go to explore the castle.
The game is best played with a friend, but you can also play it alone, controlling both Halli-Valli and Huuma-Tuuma.
The idea is to lead the two characters to their own doors to exit the room.
The game page: https://pikku-a.itch.io/the-unknown-castle-demo