Castelian is an action-platformer-puzzle game, which is a port of the computer game Tower Toppler (also known as Nebulus). The game's title is a play on the word for the European Spanish language Castillian. In this game, you play as Julius, the 'Anurian Hybrid'.
Your character Julius lives in the town of Jemmerville where there are many types of precious diamonds. One day in his sleep, a hologram tells him that the governor witnessed seven towers built around the city by unknown aliens, and that if another one is built, the aliens will have taken over the city. As Julius wakes up and starts to say something, the hologram disappears. It is Julius' job to get to the top of each tower and demolish it once he reaches the top. The time limit is your worst enemy, if you make even very little mistakes, you won't get to the top in time. You must avoid the Roving Hexalons, which spawn at random points and try to knock you off the tower. Julius can also be defeated if he falls off the tower into the clouds below. While Julius doesn't have a health bar, he can still be defeated by either falling off the edge, or by running out of time.
The game is known for its pseudo-3D effects, but the game received a bad reputation from critics due to its ridiculous difficulty. Even the first level is really tough. Castelian was later ported to the Game Boy.