During a short but extremely prolific period in the 1980s, he created several widely-known Commodore 64 games from scratch for the Interceptor Micros Players label while still a teenager at school, including Velocipede II (1986) and Fungus (1986).
At this point, his Commodore 64 was no longer working properly, causing him to exit game development at the time, as he could not afford to replace it.
Additionally, he created a number of browser-based Java applet video game clones for his website, Java on the Brain, and one of the world's first online poker games, handling all its coding and graphics, which he sold for £400 million, receiving 12.3% of the sale price, enough to make him independently wealthy.
[1] Hörnell got the assistance of classmates to develop his games; F. Wootz and B. Eklund would type in hexadecimal due to Karl's code being written on paper first.
Major companies offered to purchase the game, allowing his family to sell it in 2004 for a large sum of money, £400 million, from which he received 12.3%.
He continued developing games for iOS under Eweguo AB, Don's Dugout (2017), Stratosphere: Gravity Rush (2018), Rail Shuffle (2019), Inbread, and Psychophant.
[citation needed] Hörnell cited Dig Dug and Paradroid as two of his favorite games of all time, and ones that had an influential effect on his development.