You still need to own the original game to play it. To keep the rights holder happy we check if you have a DK version installed or on CD. The big change is tha

You still need to own the original game to play it. To keep the rights holder happy we check if you have a DK version installed or on CD. The big change is that the game does not use the functions from the original game to run anymore. All previous versions used a dll made from the original game as a crutch, so players would have the full experience on a partly made remake. This made the game fully playable from day 1, but it also meant we were limited in a big way in what kind of changes we could make. For example if Dungeon Keeper believed there were only 32 types of creatures, KeeperFX could not push anything related to a 33th creature type to the original DLL which meant we just had 32 creature types too. Now the number of creatures types comes from a user editable config file. tldr: KeeperFX is there to make DK better, it does not make free.