Castelian is a puzzle platformer in which the basic goal is to ascend towers from the base to the apex. Poro, your frog-like character, is tasked with demolishing these towers by planting a bomb at each tower's top after finding the right route up the tower.

The game was also released on other systems as Nebulus, Kyoro-chan Land, Subline, and Tower Toppler. Nebulus is the original name of the game when it was first developed for the Commodore 64 and ZX Spectrum. Kyoro-chan Land is a licensed reskin starring Kyoro-chan, a mascot for a Japanese chocolate brand named ChocoBall, with Kyoro-chan replacing Poro and ChocoBalls replacing jewels.

ktwo uses the Japanese version of the game as with his previous run and improves on it by around 42 seconds thanks to the use of alternate exits found on various levels.