Until that point the company had only developed websites and multi-media installations. Kerbal wasn't even an official company initiative, it started as a side project by one of their employees, Felipe Falanghe. The sandbox game allowed players to cobble together rockets from an inventory of modular components and attempt to put them into orbit around the planet Kerbin. It was immediately addictive.
There was no story to follow, or enemies to battle. The closest thing to a score counter was the altimeter that showed how far your craft was above the planet's surface, and the only way to "win" was to put its little green occupant, the titular Kerbal, back on the ground in one piece. The game's challenge came not from puzzles or scripted events, but from the game's accurate (if slightly simplified) application of orbital mechanics and Newtonian dynamics. Building a rocket and getting it into orbit in KSP isn't difficult because the developers baked some arbitrary limitations into their virtual world; the game is hard for the same reasons putting a rocket into orbit around the Earth is hard.