From the back of the book:
Tired of doing all the work for the group you adventure with? Now you can play too. Castle Thrax does the work for you. A revolutionary concept in Fantasy Role-Playing modules is designed to be played without a dungeon master! No referee of any kind! One person is assigned to simply read the descriptions and instructions from this unique adventure packet. No dungeon mastering experience is necessary.
Sure, being the dungeon master is fun, but not all the time. Castle Trax gives you the opportunity to adventure with your friends, meet the same challenges, slay the same vile threats to humanity. And gain experience points right along with them.
Castle Thrax has other playing opportunities. Perhaps your friends live a great distance from you or they don't get together to play as often as you like. Get out your favorite character and some NPC's and play Castle Thrax solo. There's no reason to be left behind as your friends' characters gain increasingly higher levels because they play more often than you.