Castelian is based off the original arcade game Nebulus. It was develped by Bite Studios and released in North America by Triffix and by Hiro Entertainment in Japan, in 1991.

Castelian is a platform game with some distinctive unique features. The player character, a small green creature called Pogo, is on a mission to destroy eight towers that have been built in the sea, by planting bombs at the towers' peaks. Pogo's progress is hindered by enemies and obstacles, which he has to avoid in order to reach the top of the tower.

The actual game play happens at each tower in turn. Pogo starts from the bottom and has to find his way up to the top. The towers are cylinder-shaped and have ledges on their outside, either horizontal, forming stairs or connected by elevators. Because of the cylindrical shape, the towers have no "left" or "right" edges, instead allowing Pogo to walk all around the tower.

A graphical innovation, and perhaps the most notable feature of the game, is that when Pogo walks left or right, he always stays in the centre of the visible screen. Instead of the Pogo sprite moving, the tower behind him turns clockwise or counterclockwise with a convincing sense of depth. This was featured favourably in reviews of the game.

Along his way to the top of the tower, Pogo encounters many different enemies, mostly shaped like basic geometric shapes. Pogo can shoot some of the enemies, while some are impervious to shooting. Contact with an enemy knocks Pogo down to the ledge below. If there is no ledge below, Pogo falls into the sea and drowns.

Julius is his name. Demolition is his game. Eight colossal towers that almost crack the sky! You and Julius - the Anurian Hybrid - are charged with the task of collapsing the alien towers. As Julius climbs and leaps across the face of each tower, he is confronted by potent and very deadly adversaries. Roving Hexalons, scanning I-Balls, orbiting Spheroids and dissolving pathways threaten you at every turn. Their mere touch will cost you your life! Yet, somehow, you must reach the tower's top in order to detonate your powerful D-bombs! Your courage, skill and determination will guide you to the summit of Castelian. Fire your Carbonobombs, catch the high-speed elevator, and escape into the doorways of the unknown. The island's future depends upon you and Julius. Watch him, learn his secrets and demolition will become your game!

The player character, a small green creature called Pogo, is on a mission to destroy eight towers that have been built onto the sea, by planting bombs at the towers' peaks. Pogo's progress is hindered by enemies and obstacles, which he has to avoid in order to reach the top of the tower.

The actual game play happens at each tower in turn. Pogo starts from the bottom and has to find his way up to the top. The towers are cylinder-shaped and have ledges on their outside, either horizontal, forming stairs or connected by elevators. Because of the cylindrical shape, the towers have no "left" or "right" edges, instead allowing Pogo to walk all around the tower.

Nebulus is an Amiga miscellaneous arcade game released in 1988 by Epyx.

A serious problem is blocking attempts to colonize the otherwise life-compatible planet Nebulus: it's water is poisonous! The only way to return the water to its harmless, natural form is to destroy the eight enormous towers that maintain a mysterious, lethal hold over all forms of this valuable resource.
Spurred to action by the needs of your dangerously overpopulated home planet, you must attempt to conquer these alien-forsaken relics once and for all. But it won't be easy! You'll have to climb each tower all the way to the top in order to detonate its vulnerable spot. And, succeeding towers are more difficult to topple!
You'll need all your wits about you to avoid huge, crushing boulders, giant eyes, and indestructible mutant molecules! And watch out for those slippery stairs and disintegrating ledges. One false step and you'll plummet to the depths below!