It was made free to play as part of the 20th year anniversary celebration of the original Wolfenstein game. Your goal in Wolfenstien 3D is to escape attacks by soldiers and dogs using your rifle and escape from Castle Wolfenstein. The graphics seem prehistoric nowadays but the general atmosphere and gameplay still retains the original appeal. It's still a very addictive game which is why it continues to be hugely popular and best of all, it's now free to play online.
Wolfenstein 3D online retains the addictive ingredients of the original.