Ancipital was a game released by Llamasoft in the 80's. Written by Yak (aka Jeff Minter) for the Commodore 64, it displayed his usual innovation and humour, as well as his innate sense of good gameplay.
You were the eponymous Ancipital, and had to make your way through 100 rooms. Each room had different enemies, but the rules for each room where different - reinforcing Yak's reputation for creating a more intelligent form of computer game - you had to think, or you would not survive. A superb later example of this is Iridis Alpha.
If you have never come across Llamasoft, Yak, Ancipital or any of his games, you are strongly urged to grab yourself an emulator (WinVICE works well), grab some images of the games (Yak has released them into the public domain), and discover some vintage gems...